fix(kap-server): use HTTP error statuses for session export

[wbxl2000]

Related Issue

Follow-up to #1646, which added the Web session export endpoint and has since merged. This follow-up closes the serializer/schema gap found during review.

Problem

The export route declared a binary-only 200 response while validation and export failures also returned JSON envelopes with HTTP 200. Fastify could therefore apply the binary response contract to an error object, leaving the Web client with an opaque or malformed failure.

What changed

• Return validation failures as HTTP 400, missing sessions as 404, archive-size failures as 413, and unexpected export failures as 500.
• Keep the successful response as a streamed application/zip and declare the JSON error responses separately in the route/OpenAPI contract.
• Preserve the historical 200/envelope behavior for other routes through an opt-in validation status on defineRoute.
• Add HTTP-level regression coverage and update the Web client error mock.

Verification

• pnpm exec vitest run test/sessions.test.ts test/openapi.test.ts --maxWorkers=1 in packages/kap-server (57 tests)
• pnpm exec vitest run test/apiSurface.snapshot.test.ts --maxWorkers=1 in packages/kap-server
• pnpm exec vitest run test/daemon-client.test.ts --maxWorkers=1 in apps/kimi-web (8 tests)
• pnpm --filter @moonshot-ai/kap-server run typecheck
• pnpm --filter @moonshot-ai/kimi-web run typecheck
• git diff --check

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.

[changeset-bot]

🦋 Changeset detected

Latest commit: dc03712

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

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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[wbxl2000]

@codex review

[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@moonshot-ai/kimi-code@dc03712

npx https://pkg.pr.new/@moonshot-ai/kimi-code@dc03712

commit: dc03712

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

Reviewed commit: dc03712388

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