fix(session): preserve reasoning across empty tool-call deltas

[yixiao5428]

Issue for this PR

Closes #47875

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Backports vercel/ai#20253, released in compatible 3.0.44, to OpenCode's pinned @ai-sdk/openai-compatible@2.0.41.

The parser currently ends active reasoning for tool_calls: [], splitting consecutive reasoning deltas into separate persisted parts. Requiring a nonempty array before entering the tool branch removes that false boundary while preserving real text/tool boundaries and normal stream completion.

• Keeps the current SDK interface, updating source, ESM and CJS in the existing patch and retaining its structured-error fix.
• Preserves genuine text/tool boundaries without content-rewriting heuristics or processor coalescing. No dependency upgrade or processor, adapter or UI production-code changes.
• Adds 26 tests through the real SDK, adapter and persisted session parts, including error/interruption cleanup.

Related work and differences

• #32152 and #32148, both closed without merging, group reasoning and strip echoed text in the TUI. This backport instead prevents the empty-array split before persistence, fixing stream events and stored parts rather than their display alone.
• vercel/ai#20251 contains the upstream reproduction, not a production fix. The implementation credited above is fix: prevent empty tool-call deltas from fragmenting OpenAI-compatible reasoning streams vercel/ai#20253.

This targets the empty-array case in #47875, not every symptom in #40865 or general reasoning/text coalescing.

How did you verify your code works?

• Confirmed six regression failures before the fix; the same 15-test regression/control set passed afterward with unchanged assertions. Expanded coverage: 26/26 passed.
• Full session/permission/provider run: 1,258 passed, 0 failed; 7 existing skips and 1 todo.
• Package typecheck passed; forced workspace typecheck passed all 30 tasks.
• Verified three-target patch application, actual Node ESM/CJS entrypoints, formatting and whitespace.

Commands below use workspace-relative paths. Validation ran in a source tree matching this commit, installed with bun install --frozen-lockfile:

env -u ANTHROPIC_BASE_URL bun test --cwd packages/opencode --timeout 30000 test/session test/permission test/provider
bun run --cwd packages/opencode typecheck
bun run typecheck --force

The test subprocess unsets an inherited custom Anthropic endpoint that interferes with the Gateway routing fixtures. No tests or user configuration were changed to accommodate it.

Screenshots / recordings

N/A — no UI code changes; regression coverage verifies stream events and persisted reasoning parts.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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