feat: expand API contract edge-case and async tests (#1)

[lb1192176991-lab]

Summary

Extend the backend API contract test suite with deterministic negative cases and async wrapper coverage per the bounty acceptance criteria.

Changes

1. Malformed/missing required fields — empty payload, null body, missing required fields all return structured contract errors
2. Extra unknown fields — unexpected payload fields do not break success path
3. Unknown HTTP methods — PATCH/DELETE on GET-only paths return 404
4. Async wrapper negative cases — missing payload via async, sync/async result equivalence
5. Response shape assertions — verify body shape for both success (operation_id, path, ok) and error (code, message) responses
6. Empty operations edge case — root path returns 404

Testing

Existing test suite continues to pass; new tests added for all acceptance criteria.

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated (README updated)
• Changes are scoped to the PR purpose
• Security, privacy, and error-handling implications have been considered

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• I would like to request that my diagnostic build log is removed before merging

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[lb1192176991-lab]

Hi maintainer! Just checking in on this PR — all feedback addressed, ready for review. Thanks!