fix(N+1): Batch fetch testruns in ProcessFlakesTask

[sentry]

This PR addresses an N+1 query issue identified in the app.tasks.flakes.ProcessFlakesTask, specifically within the process_flakes_for_commit function in apps/worker/services/test_analytics/ta_process_flakes.py.

Problem:
The process_flakes_for_commit function iterated over a list of uploads and, for each upload, called process_single_upload. Inside process_single_upload, get_testruns(upload) was invoked, which executed a separate database query to fetch testruns for that specific upload. This resulted in 1 + N database queries (1 for uploads, N for testruns) for each commit being processed.

Solution:
To eliminate the N+1 query, the following changes were implemented:

1. A new function, get_testruns_for_uploads, was introduced to fetch all relevant testruns for a list of upload_ids in a single batched database query. The results are then grouped by upload_id.
2. The process_single_upload function was refactored to accept a pre-fetched list of Testrun objects directly, removing its internal database call.
3. In process_flakes_for_commit, uploads are now evaluated to a list, and get_testruns_for_uploads is called once with all upload_ids. The grouped testruns are then passed to process_single_upload within the loop.
4. The old get_testruns function and an unused QuerySet import were removed.

This change reduces the database query count from 1 + N to a constant 2 queries per commit, significantly improving performance for commits with many associated uploads.

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Fixes WORKER-YQ6

[sentry]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.90%. Comparing base (508927b) to head (c8e42c6).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1029   +/-   ##
=======================================
  Coverage   91.90%   91.90%           
=======================================
  Files        1318     1318           
  Lines       50711    50714    +3     
  Branches     1625     1625           
=======================================
+ Hits        46604    46607    +3     
  Misses       3801     3801           
  Partials      306      306           

Flag	Coverage Δ
workerintegration	58.55% <11.11%> (-0.02%)	⬇️
workerunit	90.35% <100.00%> (+<0.01%)	⬆️

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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