test: increase code coverage to 100% (#14)

Add comprehensive tests covering all code paths:
- All _normalize_* helper edge cases (non-dict, non-string, empty,
  non-existent, non-directory, non-file inputs)
- _working_directory with real path (chdir/restore)
- _validate_message, _validate_branch real engine calls
- _validate_push, _validate_author, _validate_all internal branches
- All MCP tool error paths (empty/none values)
- validate_repository_state with all flag combinations
- server_health, describe_validation_rules edge cases
- main() entry point via patched mcp.run

65 tests total, 100% statement coverage on server.py

Author: shenxianpeng