perf(worker): eliminate N+1 query in commit parent lookup

[sentry]

This PR refactors the fetch_appropriate_parent_for_commit function in apps/worker/services/repository.py to address an N+1 query issue.

Problem:
The original implementation performed up to two database queries for each BFS level of a commit's ancestor tree. For repositories with deep commit histories, this resulted in a large number of redundant database calls (e.g., ~60 queries per task invocation observed in WORKER-SM9), leading to significant performance overhead.

Solution:

1. The function now first traverses the entire in-memory ancestor tree to collect all relevant commit IDs.
2. A single batched database query is then executed to fetch the commitid, branch, and message status for all collected ancestor IDs.
3. An in-memory lookup map is created from the query results.
4. The logic for selecting the closest parent (with and without a message, including branch tie-breaking) is then replayed entirely in-memory using this map, preserving the original behavior.

This change drastically reduces the number of database queries from N+1 to a single query, improving the performance of the CommitUpdate task.

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Fixes WORKER-SM9

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[thomasrockhu-codecov]

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✅ Risk: LOW

This PR refactors fetch_appropriate_parent_for_commit in apps/worker/services/repository.py to eliminate an N+1 query, replacing per-BFS-level database queries with a single batched query and an in-memory replay of the closest-parent selection. The refactor is behaviorally equivalent to the original: BFS levels are collected in the same order, the single query fetches the same (commitid, branch, message) set filtered by repo and non-deleted, and the new _possibly_filter_out_branch_from_list mirrors _possibly_filter_out_branch (single-candidate short-circuit, then first branch match, else None). Message-existence semantics and the closest_parent_without_message fallback are preserved, and existing BFS-path tests continue to cover the changed code.

No issues found above the confidence threshold.

[codecov-notifications]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
apps/worker/services/repository.py	92.85%	2 Missing ⚠️

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[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.63%. Comparing base (17e0f08) to head (b658a30).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
apps/worker/services/repository.py	92.85%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2227      +/-   ##
==========================================
- Coverage   91.63%   91.63%   -0.01%     
==========================================
  Files        1337     1337              
  Lines       53262    53285      +23     
  Branches     1647     1647              
==========================================
+ Hits        48805    48826      +21     
- Misses       4136     4138       +2     
  Partials      321      321              

Flag	Coverage Δ
workerintegration	58.51% <75.00%> (+0.01%)	⬆️
workerunit	90.55% <92.85%> (+<0.01%)	⬆️

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