fix(bigquery-analytics): annotate content_parts to satisfy mypy-diff

The skip-parse branch assigned content_parts from a bare [] inside tuple
unpacking, which mypy could not infer (var-annotated error on 3.10-3.13).
Annotate content_json/content_parts/parser_truncated before the branch.

Author: caohy1988

src/google/adk/plugins/bigquery_agent_analytics_plugin.py

@@ -3436,6 +3436,9 @@ async def _log_event(
     # #321: when both payload columns are projected out, skip content parsing
     # entirely -- no inline summary, no parts, and (critically) no GCS offload
     # work for a row that retains neither payload column.
+    content_json: Any
+    content_parts: list[dict[str, Any]]
+    parser_truncated: bool
     if {"content", "content_parts"} <= self._denied_columns:
       content_json, content_parts, parser_truncated = None, [], False
     else: