Refactor: Improve modularity and fix bugs in check_codeql_alerts.py

Refactor the `check_codeql_alerts.py` script to improve its structure and remove all uses of `locals()`.

- The large `main()` function has been broken down into smaller, more focused helper functions (`_compare_alerts_via_api` and `_build_multi_section_comment`).
- A new `APIAlertComparison` dataclass is used to provide a clear data structure for API comparison results.
- This refactoring eliminates the need for `locals()` by ensuring variables are properly initialized and passed between functions.

Fixes the following bugs introduced during the refactoring:
- A `NameError` caused by incorrect class definition order.
- A regression where matched alerts were no longer reported in the summary output.
- A regression in the logic for when to post a PR comment.

Author: google-labs-jules[bot]

scripts/check_codeql_alerts.py

@@ -15,21 +15,6 @@
 from typing import Any
 
 
-@dataclass
-class APIAlertComparison:
-    """Holds the results of comparing alerts between refs via the API."""
-
-    new_alerts: list[Alert]
-    fixed_alerts: list[Alert]
-    matched_alerts: list[Alert]
-    new_vs_prev: list[Alert]
-    fixed_vs_prev: list[Alert]
-    new_vs_base: list[Alert]
-    fixed_vs_base: list[Alert]
-    base_sha: str | None
-    prev_commit_ref: str | None
-
-
 class GitHubAPIError(RuntimeError):
     """Raised when the GitHub API returns an unexpected response."""
 
@@ -117,6 +102,21 @@ def severity_suffix(self) -> str:
         return ""
 
 
+@dataclass
+class APIAlertComparison:
+    """Holds the results of comparing alerts between refs via the API."""
+
+    new_alerts: list[Alert]
+    fixed_alerts: list[Alert]
+    matched_alerts: list[Alert]
+    new_vs_prev: list[Alert]
+    fixed_vs_prev: list[Alert]
+    new_vs_base: list[Alert]
+    fixed_vs_base: list[Alert]
+    base_sha: str | None
+    prev_commit_ref: str | None
+
+
 def parse_args(argv: collections.abc.Sequence[str] | None = None) -> argparse.Namespace:
     parser = argparse.ArgumentParser(description=__doc__)
     parser.add_argument(