refactor: optimize API test suite quality and coverage

[GoBeromsu]

Summary

• Optimized API test suite by removing trivial and redundant tests
• Enhanced focus on business logic validation and error handling
• Improved test quality while reducing code size by 81 lines (-17%)

Changes Made

prompt.test.ts

Removed:

• Tests checking constant string content (DEFAULT_SYSTEM_ROLE, DEFAULT_TASK_TEMPLATE)
• Redundant tests verifying individual placeholder replacements
• Test verifying JavaScript's .join() method behavior

Enhanced:

• Consolidated placeholder replacement into comprehensive test
• Added edge case testing (empty arrays, special characters)
• Improved custom template validation

UnifiedProvider.test.ts

Removed:

• Trivial header structure tests
• Parametric test covering all providers with minimal assertions

Enhanced:

• Added comprehensive error handling tests for each provider
• Added validation for missing/malformed response content
• Added edge case tests for invalid JSON and missing required fields
• Added integration tests for provider-specific behaviors (Anthropic, Gemini)
• Added test for temperature override logic

index.test.ts

Removed:

• Singleton pattern implementation test (language feature)
• Delegation tests redundant with integration tests
• Mock-heavy tests with low business value

Enhanced:

• Comprehensive HTTP error status code testing (400, 401, 404, 429, 500, 503)
• Native error rethrowing validation
• Non-Error exception conversion testing

Test Metrics

• Before: 31 tests, 272 lines removed
• After: 24 tests, 191 lines added
• Net Change: -7 tests, -81 lines (-17% code reduction)
• Coverage: Maintained while improving quality

Quality Improvements

1. Tests now focus on business logic, not implementation details
2. Better error case coverage across all HTTP status codes
3. More realistic provider integration testing
4. Eliminated tests of language features and constants
5. Reduced maintenance burden through consolidation

Test Results

PASS __tests__/api/prompt.test.ts
PASS __tests__/api/UnifiedProvider.test.ts
PASS __tests__/api/index.test.ts

Test Suites: 8 passed, 8 total
Tests:       55 passed, 55 total

🤖 Generated with Claude Code