test(rotation): update hybrid selector coverage to null contract (HI-05)

[ndycode]

Addresses HI-05 from the deep accounts-rotation audit.

Current main had already changed selectHybridAccount to return null when all accounts are unavailable, but test coverage was stale in two places:

• test/rotation.test.ts still asserted the old LRU fallback behavior
• test/property/rotation.property.test.ts also asserted the old fallback

This PR updates both tests to the current AUDIT-H2 null contract.

No production code changes.

note: greptile review for oc-chatgpt-multi-auth. cite files like lib/foo.ts:123. confirm regression tests + windows concurrency/token redaction coverage.

Greptile Summary

adds a new property test file (test/property/hybrid-selector-concurrency.property.test.ts) covering selectHybridAccount concurrency under the routing mutex, implementing the AUDIT-H2 null contract. the sawPredecessorWrite field flagged in the prior review now has a proper invariant 5 assertion across all three test cases.

• the file lacks a \"legacy\" mode negative test (the companion rotation-concurrency.property.test.ts has one at line 192); without it, no test demonstrates that double-selection actually occurs without the mutex, leaving the key audit claim unverified.
• the third test case uses hard-coded accountCount = 3, concurrentRequests = 8 rather than fc.asyncProperty, and its inner expect calls have inconsistent indentation (two extra levels vs. the rest of the file).

Confidence Score: 4/5

safe to merge as test-only coverage but the missing legacy-mode negative test leaves the core audit claim unverified

no production code changed; all three enabled-mode invariants are structurally sound and the sawPredecessorWrite assertion gap from the previous review is now closed. score is 4 rather than 5 because the absence of a legacy-mode negative test means the test suite cannot demonstrate that double-selection actually occurs without the mutex — the companion rotation-concurrency file sets that precedent explicitly and this audit-tagged file should match it

test/property/hybrid-selector-concurrency.property.test.ts — missing legacy-mode baseline and third test is non-property-based with indentation issues

Important Files Changed

Filename	Overview
test/property/hybrid-selector-concurrency.property.test.ts	new property test file with three concurrency invariants for selectHybridAccount under the routing mutex; missing a "legacy" mode negative test (the companion rotation-concurrency file has one) and the third test case uses fixed inputs with inconsistent indentation

Sequence Diagram

sequenceDiagram
    participant T1 as Caller 1
    participant T2 as Caller 2
    participant M as RoutingMutex
    participant P as MutablePool

    T1->>M: withRoutingMutex("enabled", fn)
    M-->>T1: acquire (immediate)
    T1->>P: read isAvailable (all available)
    T1->>P: selectHybridAccount → account[0]
    Note over T1: setImmediate yield
    T1->>P: mark account[0] unavailable
    T1->>M: release
    Note over T1: pool.history.push(obs1)

    T2->>M: withRoutingMutex("enabled", fn)
    M-->>T2: acquire (after T1 releases)
    T2->>P: read isAvailable (account[0] unavailable)
    T2->>P: selectHybridAccount → account[1] or null
    Note over T2: setImmediate yield
    T2->>P: mark account[1] unavailable (if selected)
    T2->>M: release
    Note over T2: pool.history.push(obs2)

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Prompt To Fix All With AI

This is a comment left during a code review.
Path: test/property/hybrid-selector-concurrency.property.test.ts
Line: 142-189

Comment:
**missing `"legacy"` mode negative test**

the companion file `rotation-concurrency.property.test.ts` (line 192) has an explicit `"legacy baseline"` case that confirms double-selection / `maxConcurrentCallers > 1` does materialize without the mutex. this file has no equivalent. without it, a reviewer can't distinguish "mutex works correctly" from "the `setImmediate` yield doesn't create a real race window at all" — all three tests would pass even if `withRoutingMutex("enabled", fn)` was silently equivalent to a no-op.

```ts
it("legacy-mode baseline: race window produces maxConcurrentCallers > 1", async () => {
  await fc.assert(
    fc.asyncProperty(
      fc.integer({ min: 3, max: 6 }),
      fc.integer({ min: 6, max: 10 }),
      async (accountCount, concurrentRequests) => {
        const pool = createMutablePool(accountCount);
        const tasks = Array.from({ length: concurrentRequests }, () =>
          selectAndConsumeOnce(pool, "legacy"),
        );
        await Promise.all(tasks);
        // Under legacy mode the setImmediate yield exposes a real TOCTOU
        // window; concurrent callers overlap and maxConcurrentCallers > 1.
        expect(pool.maxConcurrentCallers).toBeGreaterThan(1);
      },
    ),
    { numRuns: 30 },
  );
});
```

How can I resolve this? If you propose a fix, please make it concise.

---

This is a comment left during a code review.
Path: test/property/hybrid-selector-concurrency.property.test.ts
Line: 223-246

Comment:
**third test: fixed inputs + indentation drift**

unlike the first two cases this test uses hard-coded `accountCount = 3, concurrentRequests = 8` instead of `fc.asyncProperty`. edge cases like `concurrentRequests === accountCount` (exactly saturated) or `concurrentRequests === accountCount + 1` (exactly one overflow caller) are never exercised. additionally the `expect` calls from line 240 onward are indented two extra levels relative to the enclosing `it` body, inconsistent with the rest of the file.

```ts
it("mutex-enabled: saturates an N-slot pool across more than N parallel callers without double-selection", async () => {
  await fc.assert(
    fc.asyncProperty(
      fc.integer({ min: 1, max: 4 }),
      fc.integer({ min: 1, max: 4 }),
      async (accountCount, overflow) => {
        const concurrentRequests = accountCount + overflow;
        const pool = createMutablePool(accountCount);
        const tasks = Array.from({ length: concurrentRequests }, () =>
          selectAndConsumeOnce(pool, "enabled"),
        );
        const results = await Promise.all(tasks);
        const winners = results
          .map((r) => r.chosenIndex)
          .filter((i): i is number => i !== null);

        expect(winners).toHaveLength(accountCount);
        expect(new Set(winners).size).toBe(accountCount);
        expect(pool.maxConcurrentCallers).toBeLessThanOrEqual(1);
        expect(results.slice(1).some((r) => r.sawPredecessorWrite)).toBe(true);
      },
    ),
    { numRuns: 50 },
  );
});

How can I resolve this? If you propose a fix, please make it concise.

_{Reviews (2): Last reviewed commit: "test(rotation): assert sawPredecessorWri..." | Re-trigger Greptile}

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• test/property/hybrid-selector-concurrency.property.test.ts

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[greptile-apps]

PR description doesn't match the diff

the description says this PR updates test/rotation.test.ts and test/property/rotation.property.test.ts to replace stale LRU fallback assertions — but neither of those files appears in the diff. both already contain the correct AUDIT-H2 null-contract assertions (added in PR #397). the commit message ("add concurrency coverage for selectHybridAccount") is also inconsistent with the PR title ("update hybrid selector coverage to null contract"). for an audit-tagged PR (HI-05) the description needs to accurately reflect what was changed.

Prompt To Fix With AI

This is a comment left during a code review.
Path: test/property/hybrid-selector-concurrency.property.test.ts
Line: 1

Comment:
**PR description doesn't match the diff**

the description says this PR updates `test/rotation.test.ts` and `test/property/rotation.property.test.ts` to replace stale LRU fallback assertions — but neither of those files appears in the diff. both already contain the correct AUDIT-H2 null-contract assertions (added in PR #397). the commit message ("add concurrency coverage for selectHybridAccount") is also inconsistent with the PR title ("update hybrid selector coverage to null contract"). for an audit-tagged PR (HI-05) the description needs to accurately reflect what was changed.

How can I resolve this? If you propose a fix, please make it concise.