Expand backend API contract edge-case tests

[felixvippp-ai]

Summary

Closes #1

Expands backend API contract edge-case coverage and makes the offline mock client return structured validation errors for missing request bodies, missing required fields, and malformed field types.

Changes

• Added schema-aware request payload validation for required fields and JSON Schema primitive types.
• Added deterministic negative tests for missing required fields and malformed field types.
• Added async negative-path coverage with asyncio.run, so the suite does not require optional pytest async plugins.
• Documented the focused local test command in tests/backend_api/README.md.
• Committed diagnostic/build-011bb3d4.json, generated by build.py after the repo encryptly preflight failed on this Windows environment.

Testing

• .\.venv\Scripts\python.exe -m pytest -q tests\backend_api -> 15 passed.
• .\.venv\Scripts\python.exe -m py_compile backend\api_contract.py tests\conftest.py tests\backend_api\test_openapi_contract.py -> passed.
• python build.py -> failed during encryptly preflight before module builds; it generated diagnostic/build-011bb3d4.json explaining why no encrypted .logd was created.

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• 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

Bounty payout address: 0x1aba67a6f349826fbaea17f54556a42d42399474

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