perf: Optimize N+1 query in ProcessFlakesTask

[sentry]

The root cause analysis identified an N+1 query pattern within the app.tasks.flakes.ProcessFlakesTask in apps/worker/services/test_analytics/ta_process_flakes.py.

Specifically:

1. The process_flakes_for_commit function fetched uploads for a commit.
2. It then iterated over these uploads, and for each upload, process_single_upload would call get_testruns(upload), resulting in a separate database query for testruns per upload. This led to N+1 queries (1 for uploads + N for testruns).
3. Additionally, handle_pass was calling .save() individually for each expiring flake, leading to further N database writes.

This PR addresses these N+1 issues by:

• Introducing get_all_testruns to fetch all relevant testruns for all uploads in a single batched query, grouping them by upload_id.
• Modifying process_flakes_for_commit to use this new batched fetch, passing the pre-fetched testruns to process_single_upload.
• Modifying handle_pass to append expiring flakes to a list, which are then bulk-updated in process_flakes_for_commit after the loop completes.

These changes reduce the number of database queries from an N+1 pattern to a fixed number of queries, significantly improving performance for tasks processing multiple uploads and testruns.

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Fixes WORKER-YS4