fix(server): handle relay usage callback errors

[INONONO66]

Summary

• make RelayStream usage callbacks explicitly best-effort and non-blocking
• catch synchronous throws, asynchronous rejections, and non-Error failure values before logging
• add regression coverage for streaming and empty-body usage callback failures

Test plan

• bun test tests/unit/sse-line-cap.test.ts
• bun run typecheck
• bun run build
• bun test
• bun run release-check

Fixes #51

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Summary by cubic

Make RelayStream usage callbacks best-effort and non-blocking so they never delay or break streaming. All callback errors (sync, async, non-Error) are caught, normalized, and logged to avoid unhandled rejections.

• Bug Fixes
  • onUsage now returns void | Promise<void> and is executed without blocking the stream.
  • Catch and log synchronous throws and asynchronous rejections.
  • Normalize non-Error failures to Error before logging.
  • Add regression tests for streaming and empty-body cases to ensure no unhandled rejections.

^{Written for commit 73f5ce0. Summary will update on new commits.}

Review in cubic

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: b128c3ca-7fe7-4d77-b5f2-b0a814145875

📥 Commits

Reviewing files that changed from the base of the PR and between 7102acc and 73f5ce0.

📒 Files selected for processing (2)

• src/server/relay-stream.ts
• tests/unit/sse-line-cap.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/relay-stream-usage-errors

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