fix: return HTTP 200 with ERR pkt-line for git protocol errors

Previously, all git upload-pack/receive-pack failures returned HTTP 500,
but the git smart HTTP protocol requires protocol-level errors (like
"not our ref") to be returned as HTTP 200 OK with an ERR pkt-line in
the response body.

Changes:
- Add build_git_protocol_error_response() to create HTTP 200 responses
  with properly formatted ERR pkt-line ("ERR <message>\n")
- Add is_git_protocol_error() to detect protocol errors (exit code 128
  with stderr content) vs transport errors
- Update handle_upload_pack() and handle_receive_pack() to return
  protocol errors as HTTP 200 with ERR pkt-line
- Keep HTTP 500 for actual transport errors (spawn failures, I/O errors,
  signals)

This allows git clients to properly parse and display protocol error
messages instead of seeing generic HTTP 500 errors.

0 files changed, 0 insertions, 0 deletions