chore(tests): fix ruff lint violations in tests/ (#2827)

Clear pre-existing lint debt flagged by repo-wide `ruff check` (the lint
config only scopes src/, so tests/ had drifted). No behavior change.

- F401/F541: drop unused imports and redundant f-string prefixes (autofix)
- E741: rename ambiguous `l` to `ln` in comprehensions
- E702: split semicolon-joined statements onto separate lines
- F841: drop unused bindings while keeping the side-effecting calls
  (_minimal_feature, install_from_directory)

Full suite: 3344 passed, 40 skipped. ruff check (repo-wide): clean.

tests/extensions/git/test_git_extension.py

@@ -371,7 +371,7 @@ class TestCreateFeaturePowerShell:
         )
         assert result.returncode == 0, result.stderr
         # pwsh may prefix warnings to stdout; find the JSON line
-        json_line = [l for l in result.stdout.splitlines() if l.strip().startswith("{")]
+        json_line = [ln for ln in result.stdout.splitlines() if ln.strip().startswith("{")]
         assert json_line, f"No JSON in output: {result.stdout}"
         data = json.loads(json_line[-1])
         assert "BRANCH_NAME" in data