fix(gtea): ruff format + check all gtea files and bridge.py

Clears cc-ci self-test lint failures:
- ruff format: 9 files reformatted (all gtea test files + test_discovery.py)
- ruff check --fix: bridge.py UP017 (datetime.UTC alias) + 6 gtea check errors
- manifest.py B007: rename unused loop variable path → _path (no auto-fix available)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

runner/harness/manifest.py

@@ -51,7 +51,7 @@ def _pre_ops(path: str) -> list[str]:
 
 def _custom_counts(recipe: str, repo_local: str | None) -> dict[str, dict[str, int]]:
     out: dict[str, dict[str, int]] = {}
-    for source, path in discovery.custom_tests(recipe, repo_local):
+    for source, _path in discovery.custom_tests(recipe, repo_local):
         sub = "custom"
         out.setdefault(source, {}).setdefault(sub, 0)
         out[source][sub] += 1