fix(test_analytics): eliminate N+1 query in ProcessFlakesTask

[sentry]

This PR addresses an N+1 query pattern identified in the ProcessFlakesTask (app.tasks.flakes.ProcessFlakesTask).

Root Cause:

1. When processing flakes for a commit, the original implementation fetched uploads, then iterated through each upload, performing a separate database query to retrieve its associated testruns. This resulted in 1 (for uploads) + N (for testruns) queries.
2. Additionally, when a flake expired (met the criteria for 30 recent passes), it triggered an individual .save() operation, leading to multiple single-row updates.

Solution:

1. Batch Testrun Fetching: Introduced a new function get_testruns_for_uploads that fetches all relevant testruns for all uploads in a single batched query using upload_id__in. The results are then grouped by upload_id in Python.
2. Refactored Processing: The process_single_upload function was updated to accept pre-fetched testruns, eliminating the need for per-upload database calls.
3. Batch Flake Updates: Modified handle_pass to collect expired flakes into a list. Instead of individual .save() calls, process_flakes_for_commit now performs a single Flake.objects.bulk_update() operation for all expired flakes after processing all uploads.

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Fixes WORKER-YNZ