fix(worker): N+1 query in ProcessFlakesTask for testruns and expired flakes

[sentry]

This PR addresses an N+1 query issue in app.tasks.flakes.ProcessFlakesTask (apps/worker/services/test_analytics/ta_process_flakes.py).

Problem:
Previously, when processing flakes for a commit:

1. process_flakes_for_commit would fetch all relevant uploads for a commit.
2. For each upload, process_single_upload would then execute a separate database query via get_testruns(upload) to fetch its associated testruns, leading to an N+1 query pattern.
3. Additionally, handle_pass would call flake.save() individually when a flake reached 30 recent passes, potentially causing many single UPDATE statements.

Solution:

1. Bulk Fetching Testruns: The get_testruns function has been refactored into get_testruns_for_uploads to fetch all testruns for all uploads related to a commit in a single database query using upload_id__in.
2. In-Memory Grouping: These testruns are then grouped by upload_id in memory before being passed to process_single_upload.
3. Bulk Update for Expired Flakes: handle_pass no longer calls flake.save() directly. Instead, expired flakes are collected into a list and a single Flake.objects.bulk_update is performed after all uploads have been processed.

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Fixes WORKER-YNX