Optimize clang-format check in Jenkins CI

[tenpercent]

Summary

This PR optimizes the Jenkins CI clang-format check to be faster and remove the git dependency.

Changes

• Use find instead of git ls-files - No git dependency required in build environment
• Check all C++ files: *.h, *.hpp, *.cpp, *.h.in, *.hpp.in, *.cpp.in, *.cl
• Exclude directories: build/ and include/rapidjson (same as before)
• Parallel processing: Use 8 cores (-P 8) instead of sequential processing
• Cleaner output: Show only errors with unified diff format (-u)
• Better error messages: "ERROR: needs formatting"
• Preserve original logic:
  • When RUN_CPPCHECK=false: Run clang-format only
  • When RUN_CPPCHECK=true: Run both clang-format and cppcheck

Performance

Measured on 5,899 C++ files:

• Sequential processing (-P 1): ~93 seconds
• Parallel with 8 cores (-P 8): ~12 seconds
• Speedup: 7.9x faster
• Per-file processing: ~15ms

Benefits

• 8x faster clang-format checks (~81 seconds saved)
• No git dependency - works in any build environment
• Same file coverage - all C++ files are still checked
• Cleaner CI output - only shows errors, not every file processed
• Better error reporting - unified diff format shows exactly what needs fixing

Test Plan

• Verify clang-format runs successfully on all files
• Verify both RUN_CPPCHECK=true and RUN_CPPCHECK=false scenarios work
• Confirm formatting errors are properly detected and reported
• Validate parallel processing completes correctly