Increase Backend Test Coverage to 95% with Unit, Integration & Async Workflow Tests

Description

This merge request significantly improves backend test coverage from ~65% to ≥95% by adding comprehensive unit tests, integration tests, and workflow validation for critical system components.

The focus is on strengthening reliability across complex backend areas such as real-time streaming, background job processing, and AI/ML integrations. The update also ensures better handling of edge cases, failure scenarios, and asynchronous execution paths.

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What’s Included

Test Coverage Improvements

• Expanded unit tests for core business logic, utilities, and service layers
• Added integration tests for end-to-end workflows
• Covered edge cases, exception handling, and fallback mechanisms

Mocking & Isolation

• Mocked heavy external dependencies (AI/ML models, APIs)
• Reduced test execution time while maintaining reliability
• Ensured tests are deterministic and isolated

Targeted Fixes

• Improved coverage for previously untested branches
• Added tests for fallback paths and error-handling logic
• Addressed low-coverage utility modules

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Key Improvements

• Increased overall backend coverage to ≥95%
• Improved system stability and reliability
• Strengthened validation of async and streaming workflows
• Reduced risk of production failures
• Enhanced maintainability and confidence in code changes

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Acceptance Criteria

• ✅ Overall test coverage ≥95%
• ✅ All tests pass successfully
• ✅ Critical modules (services, streaming, job management) ≥90% coverage
• ✅ No significant increase in test execution time

*Closes #27 (closed)