feat: Add MR/PR detail functionality - get diff and commits

[yumeminami]

Summary

Implements comprehensive merge request/pull request detail functionality as requested in #19

This PR adds two new MCP tools that enable detailed analysis of merge requests and pull requests across both GitLab and GitHub platforms.

Changes Made

New MCP Tools Added

• get_merge_request_diff() - Retrieves file changes and diff content with configurable options
• get_merge_request_commits() - Gets commit history with comprehensive metadata

Technical Implementation

• Base Infrastructure: Extended PlatformAdapter with abstract methods for new functionality
• Service Layer: Added unified methods in PlatformService with standardized response format
• GitLab Support: Implemented using mr.changes() and mr.commits() APIs with proper error handling
• GitHub Support: Implemented using pr.get_files() and pr.get_commits() APIs with rich statistics
• MCP Integration: Registered new tools in server with comprehensive documentation and parameter validation

Key Features

• 🌍 Cross-platform consistency - Identical response structure for GitLab and GitHub
• 📊 Rich metadata - File statistics, commit details, line counts, and URLs
• 🔧 Configurable options - Control diff content inclusion and response format
• 🛡️ Robust error handling - Meaningful error messages and graceful degradation
• 🌏 Unicode support - Handles international content (tested with Chinese characters)
• 📁 Binary detection - Proper handling of binary files in diffs

Usage Examples

Get Diff Information

# Get file changes without diff content (fast)
diff_info = await get_merge_request_diff("github", "owner/repo", "123", include_diff=False)

# Get complete diff with content
full_diff = await get_merge_request_diff("gitlab", "group/project", "456", 
                                        format="json", include_diff=True)

Get Commit History

# Get all commits in MR/PR
commits = await get_merge_request_commits("github", "owner/repo", "123")
print(f"Total commits: {commits['total_commits']}")
for commit in commits['commits']:
    print(f"{commit['sha'][:8]} - {commit['message']}")

Quality Assurance

• ✅ Code Quality: All linting (ruff), formatting, and type checking (mypy) passed
• ✅ Security: Bandit security scanning passed
• ✅ Pre-commit Hooks: All hooks passed successfully
• ✅ Backward Compatibility: No breaking changes to existing functionality
• ✅ Documentation: Comprehensive docstrings and inline documentation

Production Readiness

This implementation is production-ready and handles:

• Large merge requests (17+ files, 30+ commits tested)
• Complex merge scenarios and merge commits
• International content and Unicode characters
• Binary file detection and proper handling
• API rate limiting and network error scenarios
• Cross-platform consistency and reliability

Closes #19