chore: untrack tests/failed_uuids.txt and add to .gitignore

This file is auto-generated by the test runner to track failures for CI
retry logic. It should never be committed.

Author: sbryngelson

.gitignore

@@ -116,3 +116,5 @@ run_cce_*.sh
 .ffmt_cache/
 # FP-stability log artifacts (generated by ./mfc.sh fp-stability)
 fp-stability-logs/
+# Test runner failure tracking (auto-generated, not for version control)
+tests/failed_uuids.txt