Add --date parameter to load_bigquery_stats job

[stormKx18]

This PR updates load_bigquery_stats.py to accept an optional --date flag (in YYYY-MM-DD format), allowing cluster operators and developers to execute the BigQuery stats loading job against an arbitrary target date (useful for backfilling or re-processing past stats).

Key Highlights

• Optional --date Flag: Supports loading stats for a specified date (e.g. --date 2026-06-20).
• Backward Compatible: Defaults to yesterday's date (_utc_now().date() - datetime.timedelta(days=1)), maintaining standard daily cron execution behavior.
• Design Parity: Follows the existing command-line argument pattern established in aggregate_fuzzer_stats.py.
• Unit Test Coverage: Added test_execute_with_date to verify that target partition tables ($YYYYMMDD) and GCS source URIs (/date/YYYYMMDD/) are correctly generated when a custom date is provided.
• Unit Test Refactoring: Extracted BigQuery dataset, table, and load job assertions into a shared _assert_load_calls(expected_date_str) helper method to avoid code duplication across default and date-specific test cases.

Testing Instructions

• Unit Tests:

  python butler.py py_unittest -t appengine -p load_bigquery_stats_test.py
  

• Unit Tests results:

[dylanjew]

nice! Mostly LGTM. Just one nit.

I think you can go ahead and test this on dev by running the cron job with butler. You may need to verify that the args are passed in correctly, I remember that being a bit annoying when trying to run the cron

[dylanjew]

Nice, thanks!