Improve test coverage from 76.33% to 85.92%

[Copilot]

This PR significantly improves the test coverage of the github/hooks project by adding comprehensive unit tests for previously untested components.

Coverage Improvement

• Before: 76.33% (374/490 lines)
• After: 85.92% (421/490 lines)
• Improvement: +9.59 percentage points (+47 lines covered)

New Test Coverage Added

1. App Helpers (lib/hooks/app/helpers.rb) - 55 tests

• UUID generation and uniqueness
• Request size limit enforcement with various header formats
• JSON payload parsing with symbolization options
• Dynamic handler class loading with filename conversion
• HTTP error code mapping for different exception types

2. Authentication Module (lib/hooks/app/auth/auth.rb) - 16 tests

• HMAC authentication validation
• Shared secret authentication validation
• Environment variable secret handling
• Authentication type case sensitivity
• Error handling for missing secrets and unsupported auth types

3. Default Handler (lib/hooks/handlers/default.rb) - 7 tests

• Webhook processing with various payload types (Hash, String, nil)
• Response format validation
• Timestamp generation using mocked time
• Configuration and header handling
• Inheritance verification

4. Core Logging (lib/hooks/core/log.rb) - 6 tests

• Global logger instance management
• Logger accessor methods
• Support for different logger implementations
• Module structure validation

5. Health Endpoint (lib/hooks/app/endpoints/health.rb) - 7 tests

• HTTP 200 status response
• JSON content type validation
• Health status information structure
• Uptime calculation accuracy
• Response field completeness

6. Version Endpoint (lib/hooks/app/endpoints/version.rb) - 6 tests

• HTTP 200 status response
• JSON content type validation
• Version information structure
• ISO 8601 timestamp format
• Response field validation

Test Quality Features

• Edge Case Coverage: Tests handle nil, empty, and malformed inputs
• Error Condition Testing: Validates proper error handling and status codes
• Mock Management: Proper test isolation to prevent mock leakage
• API Contract Testing: Ensures endpoints return expected response formats
• Cross-Platform Compatibility: Tests work with different header name variations

Technical Notes

• All tests follow existing project patterns and use RSpec best practices
• Tests are organized by module/class structure for maintainability
• Mock usage is minimal and properly isolated to avoid test interference
• Coverage improvements focus on business logic and public APIs

The test suite now provides comprehensive validation of core functionality while maintaining the existing 70% coverage requirement with significant margin.

Fixes #7.