[BE-TEST-018] be-test: Add tests for error handling
- Created comprehensive error handling test suite
- Tests verify error response format standardization
- Tests cover all error types (validation, not found, unauthorized, forbidden, internal, database, conflict, rate limit, quota)
- Tests verify error recovery and retry logic
- Tests verify validation error details
- Tests verify HTTP status code mapping
- Tests verify error response consistency
Phase: PHASE-5
Priority: P2
Progress: 139/267 (52.06%)
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