fix(perf): Eliminate N+1 queries in ProcessFlakesTask

[sentry]

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

Problem:
Previously, the process_flakes_for_commit function would iterate over each upload associated with a commit. For every upload, it would perform separate database SELECT queries to fetch Testrun objects and then separate UPDATE queries via bulk_update on the ta_timeseries_testrun table. This led to an N+1 query pattern, where N is the number of uploads for a given commit, causing significant performance overhead.

Solution:
This change refactors the process_flakes_for_commit and process_single_upload functions to batch database operations:

1. get_testruns was modified to accept a list of ReportSession objects and now fetches all relevant Testrun objects for all uploads in a single SELECT query using upload_id__in.
2. The fetched Testrun objects are then grouped by upload_id in memory.
3. process_single_upload was updated to receive a pre-filtered list of Testrun objects, removing its internal database calls.
4. All modified Testrun objects from all uploads are collected, and a single Testrun.objects.bulk_update is performed after processing all uploads, eliminating the N individual UPDATE queries.

This significantly reduces the number of database round trips, improving the performance of the ProcessFlakesTask.

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Fixes WORKER-YYN

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (701e991) to head (fa0e4d4).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1414   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files        1325     1325           
  Lines       50868    50871    +3     
  Branches     1626     1626           
=======================================
+ Hits        46744    46747    +3     
  Misses       3818     3818           
  Partials      306      306           

Flag	Coverage Δ
workerintegration	58.54% <10.00%> (-0.02%)	⬇️
workerunit	90.37% <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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