perf: Fix N+1 query in ProcessFlakesTask

[sentry]

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

Problem:
Previously, process_flakes_for_commit would iterate over each ReportSession (upload) and call process_single_upload. Inside process_single_upload, get_testruns(upload) would execute a separate database query for Testrun objects for each individual upload. This resulted in an N+1 query pattern, where N is the number of uploads.
Additionally, handle_pass would call flake.save() individually for each flake that reached its expiration threshold, leading to further unnecessary database calls.

Solution:

1. Bulk Fetching Testruns: Introduced get_testruns_for_uploads to fetch all relevant Testrun objects for all uploads in a single database query. These are then grouped by upload_id in memory.
2. Refactored process_single_upload: Modified process_single_upload to accept the pre-fetched and grouped Testrun objects, eliminating the need for per-upload database queries.
3. Optimized Flake Saving: handle_pass no longer calls flake.save() individually. Instead, flakes that reach their expiration threshold are moved to a separate expired_flakes list. This list, along with the curr_flakes, is then included in the single Flake.objects.bulk_create call at the end of process_flakes_for_commit, ensuring all updates are persisted efficiently.

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-YS3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (da7c50e) to head (c899e3d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1160      +/-   ##
==========================================
- Coverage   91.89%   91.89%   -0.01%     
==========================================
  Files        1324     1324              
  Lines       50851    50859       +8     
  Branches     1626     1626              
==========================================
+ Hits        46730    46736       +6     
- Misses       3815     3817       +2     
  Partials      306      306              

Flag	Coverage Δ
workerintegration	58.53% <14.28%> (-0.03%)	⬇️
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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