perf(worker): Optimize flake processing N+1 query

[sentry]

This PR resolves an N+1 query issue within the app.tasks.flakes.ProcessFlakesTask by optimizing how Testrun objects are fetched and updated.

Problem:
Previously, the process_flakes_for_commit function iterated over uploads, and for each upload, it would call get_testruns, resulting in a separate database query for Testrun objects per upload. Additionally, Testrun.objects.bulk_update was called inside the loop, leading to multiple update queries.

Solution:

1. Batched Fetching: The get_testruns function has been refactored to accept a list of upload_ids and now uses upload_id__in to fetch all relevant Testrun objects in a single database query.
2. In-Memory Grouping: In process_flakes_for_commit, after fetching all Testrun objects, they are grouped by upload_id in memory.
3. Refactored Processing: process_single_upload now receives the pre-fetched and grouped Testrun objects directly, eliminating the need for per-upload database queries.
4. Single Bulk Update: All modifications to Testrun objects are accumulated, and Testrun.objects.bulk_update is now called only once after all uploads have been processed.

This change significantly reduces the number of database queries and improves the performance of the ProcessFlakesTask, especially for commits with many test result uploads.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. In 2022 this entity acquired Codecov and as result Sentry is going to need some rights from me in order to utilize my contributions in this PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

Fixes WORKER-YV8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (e682d07) to head (77c04e2).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1238   +/-   ##
=======================================
  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	58.55% <11.11%> (-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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