Implement generate_commit_message_parallel main function

[oleander]

Task

Create the main entry point function generate_commit_message_parallel in src/multi_step_integration.rs that coordinates the two-phase parallel processing approach.

Requirements

• Accept OpenAI client reference, model string, diff content, and optional max message length
• Return Result with final commit message
• Call corrected parse_diff function to extract individual files
• Construct vector of futures for parallel file analysis
• Use join_all from futures crate to execute analyses concurrently
• Graceful error handling:
  • Log warnings for failed individual analyses
  • Continue with successful results
  • Only fail if no analyses succeed
• Format successful results into readable enumeration
• Construct synthesis prompt with file summaries
• Send synthesis request as simple completion (no function calling)
• Extract and return final commit message
• Instrument with profile! macro for performance measurement

Success Criteria

• Demonstrates measurable performance improvements:
  • Single file: ~4 seconds (down from 4.58s)
  • Five files: ~4.3 seconds (vs ~16s sequential)
• Resilient to individual file analysis failures
• Works correctly for commits ranging from 1 to dozens of files

Part of #81