fix(server): preserve Codex reset credits during usage updates

[yashranaway]

What Changed

Preserve Codex reset credits when merging streamed rate-limit window updates. A later full account probe can still replace or remove the credits.

Why

Sparse usage updates rebuild the limits snapshot and discard the credits returned by the last probe, making reset credits disappear from Usage → Limits during a turn.

Testing

• providerUsageLimits.test.ts passes, 5/5, including a streamed-update regression.
• Server typecheck, targeted lint, and formatting pass.

Checklist

• One focused change
• Explained what changed and why
• Added regression coverage for the changed behavior
• No UI layout or animation changes

Model: GPT-6 Astra
Harness: T3 code

Note

Preserve resetCredits in provider.applyUsageLimitsUpdate

Streamed usage-limit updates were dropping the resetCredits metadata from the prior snapshot. The merger now carries forward previous.resetCredits while still applying the new window data and updated check timestamp.

^{Macroscope summarized 1050bfa.}

[macroscopeapp]

Approvability

Verdict: Approved at 1050bfa

Macroscope's review found this PR approvable — This is a narrow server-side bug fix that preserves existing Codex reset-credit metadata during streamed usage updates, with a focused regression test. Existing no-op and full-probe behavior remains unchanged, and the PR introduces no schema, deployment, security, billing, or static-analysis changes.

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