fix: normalize malformed Responses rate limit errors

[7Sageer]

Related Issue

No linked issue; this fixes malformed Responses-compatible stream error frames that currently surface as client decode failures.

Problem

Some Responses-compatible streaming gateways may emit an error frame without a top-level type, with the actual provider error nested inside a message string. Kimi Code then reports a generic Responses decode error instead of the underlying rate limit failure.

What changed

Normalize malformed gateway stream error messages that contain nested rate limit JSON into provider rate limit errors, while still rejecting malformed events that include a non-string type. Added coverage for normal response.failed rate limits, the malformed gateway frame, and the strict non-string type boundary.

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
• pnpm -C packages/kosong run typecheck

[changeset-bot]

🦋 Changeset detected

Latest commit: 7f14cf8

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

This PR includes changesets to release 2 packages

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

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[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@moonshot-ai/kimi-code@7f14cf8

npx https://pkg.pr.new/@moonshot-ai/kimi-code@7f14cf8

commit: 7f14cf8