fix(worker): Optimize ProcessFlakesTask N+1 queries for testruns and flakes

[sentry]

This PR addresses an N+1 query performance issue in the app.tasks.flakes.ProcessFlakesTask by optimizing database interactions related to Testrun and Flake objects.

Problem:
Previously, process_flakes_for_commit would iterate over each ReportSession (upload) and:

1. Call get_testruns(upload) for each upload, resulting in a separate database query for Testrun objects per upload.
2. Call Testrun.objects.bulk_update for each upload after processing, leading to multiple update operations.
3. In handle_pass, individual flake.save() calls were made when a flake expired, causing additional single-row updates.

Solution:

1. Batch Testrun Fetching: A new function get_testruns_for_uploads was introduced to fetch all relevant Testrun objects for all uploads in a single batched query. These are then grouped by upload_id in Python.
2. Consolidated Testrun Updates: process_single_upload was refactored to accept pre-fetched testruns. All modified Testrun objects are now accumulated and Testrun.objects.bulk_update is called only once after processing all uploads.
3. Optimized Flake Updates: The individual flake.save() call in handle_pass for expiring flakes has been removed. Expiring flakes are now marked with end_date and handled by the existing Flake.objects.bulk_create(update_conflicts=True) operation at the end of process_flakes_for_commit, which efficiently updates or creates flakes in bulk.

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Fixes WORKER-YTH