Add coverage heatmap visualization with --heatmap CLI flag

[Copilot]

This PR implements a new coverage heatmap report type that provides a visual graph representation of API test coverage, addressing the need for better coverage visualization and gap analysis.

🎯 What's New

Coverage Heatmap Report

Added a new --heatmap CLI flag that generates an interactive coverage visualization featuring:

• API Coverage Matrix: A grid showing API paths (rows) vs HTTP methods (columns) with color-coded coverage status
• Interactive Endpoint Details: Expandable list showing all endpoints with method badges and test coverage information
• Visual Coverage Highlighting:
  • 🟢 Green cells = Fully covered endpoints
  • 🔴 Red cells = Not covered endpoints
  • ⚪ Gray cells = Not defined in specification
• Theme Support: Toggle between light and dark themes
• Coverage Statistics: Key metrics dashboard showing total/covered endpoints and coverage percentage

🚀 Usage

# Generate heatmap visualization
swagger-coverage-cli api.yaml collection.json --heatmap --output heatmap.html

# Regular report (unchanged behavior)
swagger-coverage-cli api.yaml collection.json --output report.html

📸 Screenshots

Light Theme Matrix View:

Dark Theme Support:

🔧 Implementation Details

• New CLI Option: --heatmap flag with comprehensive help text
• Conditional Report Generation: Automatically selects heatmap vs standard report based on flag
• Interactive Features: Clickable matrix cells show detailed coverage information
• Responsive Design: Modern Material Design styling that works across screen sizes
• Backward Compatibility: All existing functionality preserved, no breaking changes

🧪 Testing

• ✅ All existing tests continue to pass (91/91)
• ✅ New comprehensive test suite for heatmap functionality (8 new tests)
• ✅ CLI integration tests validating end-to-end behavior
• ✅ Verified both heatmap and regular report generation

💡 Benefits

This visualization makes it significantly easier to:

• Identify Coverage Gaps: Quickly spot which endpoints lack test coverage
• Analyze Method Coverage: See which HTTP methods are well-tested vs neglected
• Prioritize Testing Efforts: Visual indicators help focus on high-impact areas
• Present Results: Professional, interactive reports suitable for stakeholder review

The heatmap complements the existing detailed HTML reports by providing an at-a-glance overview that's perfect for coverage analysis and gap identification in API testing workflows.

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