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!

[vitaliset]

LGTM.

I think you can go ahead and test this on dev by running the cron job with butler.

+1. Please provide some evidence that it worked on dev before merging! :)

[stormKx18]

Please see attached screenshots of the script being validated on dev.

Before running the script there was missing data on 2025-12-29 for a specific fuzzer.

---

Run script on Dev with the target date 2025-12-29

python butler.py --local-logging run -c ../clusterfuzz-config/configs/@@@@-development run_cron 
--script_args load_bigquery_stats --date 2025-12-29

---

Results after running the script:

Data was loaded for the target date 2025-12-29 as it was intended.

[dylanjew]

nit: i would add a comment or use a variable to help make it clear that this is one day before the mock in the test

[stormKx18]

Thanks for the feedback.
I have added a variable (expected_yesterday) and a comment to clarify the unit tests.