fix(performance): Resolve N+1 queries in ProcessFlakesTask

[sentry]

This PR addresses an N+1 query issue identified in app.tasks.flakes.ProcessFlakesTask.

Changes Made:

• Bulk fetching of Testruns: Previously, process_single_upload would execute a separate database query to fetch Testrun objects for each ReportSession (upload). This has been refactored to fetch all relevant Testrun objects for all uploads in a single query, then group them in memory.
• Batch updating expiring Flakes: The handle_pass function previously called flake.save() individually for each flake that reached its expiration criteria (30 recent passes). This has been changed to collect all such expiring flakes and perform a single Flake.objects.bulk_update operation at the end of process_flakes_for_commit.

Why these changes?
These changes eliminate two N+1 query patterns, significantly reducing the number of database calls and improving the performance of the ProcessFlakesTask, especially for repositories with many uploads or a high volume of expiring flakes.

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Fixes WORKER-YQ8