feat: Add Python backend API contract edge-case tests

[GautamKumarOffical]

Summary

Adds Python tests for backend API contract edge cases covering malformed payloads, async wrappers, and error response shape validation.

Changes

• Added backend/api_contract.py with request validation helpers and error response construction
• Added tests/backend_api/test_api_contract.py with 28 test cases covering:
  • Missing required fields returning structured contract errors
  • Invalid field types triggering validation errors
  • Pattern mismatch for email and symbol fields
  • Login credential validation (missing email, missing password, invalid format)
  • Order payload validation (missing symbol, invalid symbol format, invalid side/type, non-numeric quantity)
  • Error response shape assertions (code, message, request_id, details)
  • Async helper execution without requiring optional pytest plugins

Testing

• python3 -m pytest -q tests/backend_api — all 28 tests pass
• python3 build.py -m market — market module builds successfully
• Diagnostic build log committed

Checklist

• Tests pass locally
• Diagnostic build log is committed
• Fixtures isolated (no network dependencies)
• Changes are scoped to the PR purpose

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ed6e6ecd-e4d6-4d93-a09c-8636227a9220

📥 Commits

Reviewing files that changed from the base of the PR and between 2b54872 and 6c0d6e5.

📒 Files selected for processing (4)

• backend/api_contract.py
• diagnostic/build-209c3616.json
• diagnostic/build-209c3616.logd
• tests/backend_api/test_api_contract.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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