perf(worker): Optimize N+1 query in ProcessFlakesTask

[sentry]

This PR addresses an N+1 query issue within the app.tasks.flakes.ProcessFlakesTask.

Problem:
Previously, the process_flakes_for_commit function would iterate through each ReportSession (upload) and, for every upload, make a separate database query to fetch its associated Testrun objects via get_testruns(upload). This resulted in an N+1 query pattern, where N queries were executed for N uploads, leading to inefficient database usage and slower task execution.

Solution:

1. A new function, get_testruns_for_uploads, was introduced to fetch all relevant Testrun objects for a given list of uploads in a single batched query using upload_id__in. The results are then organized into a dictionary keyed by upload_id.
2. The process_single_upload function was updated to accept a pre-fetched list of Testrun objects, removing its internal database query.
3. In process_flakes_for_commit, get_testruns_for_uploads is now called once before the loop to retrieve all testruns. The loop then processes these testruns from the in-memory dictionary.
4. Additionally, all modified Testrun objects are now collected and updated in a single Testrun.objects.bulk_update call after the loop, further reducing database operations.

Impact:
This change significantly reduces the number of database queries performed by ProcessFlakesTask, improving its performance and reducing database load, especially for repositories with many uploads.

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Fixes WORKER-YYF

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (701e991) to head (f82a0cf).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1408      +/-   ##
==========================================
- Coverage   91.89%   91.89%   -0.01%     
==========================================
  Files        1325     1325              
  Lines       50868    50878      +10     
  Branches     1626     1626              
==========================================
+ Hits        46744    46752       +8     
- Misses       3818     3820       +2     
  Partials      306      306              

Flag	Coverage Δ
workerintegration	58.53% <7.14%> (-0.04%)	⬇️
workerunit	90.36% <100.00%> (-0.01%)	⬇️

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[codecov-notifications]

Codecov Report

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

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