fix(worker): Eliminate N+1 queries in ProcessFlakesTask

[sentry]

This PR addresses an N+1 query issue within the app.tasks.flakes.ProcessFlakesTask by implementing two key optimizations:

1. Batch-fetching Testruns: Previously, process_single_upload would execute a separate database query for Testrun objects for each ReportSession (upload) within a loop. This has been refactored to batch-fetch all relevant Testrun objects for all uploads in a single query using get_testruns_for_uploads before iterating through uploads. The process_single_upload function now receives pre-fetched testruns, eliminating the N+1 query pattern.
2. Bulk-updating Expired Flakes: The handle_pass function previously called flake.save() individually when a flake reached 30 recent passes. This has been changed to collect these 'expired' flakes into a list, which are then bulk-updated using Flake.objects.bulk_update in process_flakes_for_commit after all processing is complete. This reduces individual database write operations.

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Fixes WORKER-YQ7