fix: handle structured context overflow errors

[kermanx]

Related Issue

No linked issue; this addresses inconsistent context overflow handling for provider responses.

Problem

Context overflow detection used two different text matchers, so telemetry could classify the same provider error differently from the provider layer. OpenAI Responses stream failures also exposed structured context_length_exceeded errors as generic provider errors, which prevented the overflow recovery path from seeing the dedicated context overflow type.

What changed

• Reused the shared kosong context overflow status matcher from agent-core telemetry classification.
• Normalized OpenAI Responses error and response.failed events with context_length_exceeded into the dedicated context overflow provider error.
• Added regression coverage for the shared matcher and structured Responses failure path.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

Verification

• pnpm -C packages/kosong exec vitest run test/errors.test.ts test/openai-responses.test.ts
• pnpm -C packages/agent-core exec vitest run test/agent/turn.test.ts
• pnpm -C packages/kosong run typecheck
• pnpm -C packages/agent-core run typecheck
• pnpm exec oxlint --type-aware packages/kosong/src/errors.ts packages/kosong/src/index.ts packages/kosong/src/providers/openai-responses.ts packages/kosong/test/openai-responses.test.ts packages/agent-core/src/agent/turn/index.ts packages/agent-core/test/agent/turn.test.ts

[changeset-bot]

🦋 Changeset detected

Latest commit: b47d03c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@moonshot-ai/agent-core	Patch
@moonshot-ai/kimi-code	Patch
@moonshot-ai/kosong	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@moonshot-ai/kimi-code@b47d03c

npx https://pkg.pr.new/@moonshot-ai/kimi-code@b47d03c

commit: b47d03c