fix(fix): fail when Coana leaves computed fixes unapplied

[Martin Torp (mtorp)]

Part of ENG-5437. socket fix reported success whenever Coana exited 0, and local mode hardcoded fixedAll: true. So a run where Coana skipped a directory and changed nothing still printed "Finished!" and exited 0.

The matching Coana change (coana-tech/coana-package-manager#2541) lists fixes it could not apply under notApplied in its --output-file, uses the new type no-fixes-applied when nothing was applied, and exits with 5.

What changes

• Local mode: exit 5 from Coana is treated as a result, not a crash. socket fix still reads and copies the output file, then fails with "No computed fixes were applied" or "Some computed fixes were not applied". The cause lists each unapplied fix with its reason. fixedAll is false. A non-empty notApplied with exit 0 is treated the same way.
• PR mode: exit 5 logs a warning listing the unapplied fixes and falls through to the existing check for changed files. So a PR still opens for whatever was applied, and nothing opens when nothing changed. Other Coana failures are handled as before.
• Any other non-zero Coana exit is unchanged.

This is safe to merge before the Coana change ships, because the current Coana never exits 5 or writes notApplied. It takes effect once the Coana version is bumped.

Testing

New tests in handle-fix-limit.test.mts cover exit 5 with nothing applied, exit 5 with some applied (and --output-file still written), a clean run, and an unrelated Coana failure. The fix tests pass, except handle-fix-id.test.mts, which fails locally on GitHub API mocks and is untouched here.

Expected reviewer effort: High-level design decisions