Reduce over-fetching in Bugscache queries

[camd]

Summary

Bugscache.search() (backing the bug-suggestions endpoint) and the create_bug endpoint both loaded full Bugscache rows when only a subset of columns is actually used. This trims the fetched/written columns without changing behavior.

Changes

• Bugscache.search() — add .only(...) to defer modified and processed_update, which serialize() excludes anyway. The TrigramSimilarity annotation and serialize() output are unaffected.
• Bugscache.serialize() — remove the dead "jobmap" entry from exclude_fields; it's a reverse relation that model_to_dict never iterates, so it had no effect.
• create_bug — resolve the existing internal id with values_list("id", flat=True).first() instead of fetching the whole row; restrict the summary-fallback query with .only("id", "bugzilla_id", "modified"); and limit the linking save() to the two changed columns via update_fields.

Tests

Added coverage for the create_bug internal-id resolution paths (existing bug matched by bugzilla_id, and an internal bug linked by summary), which were previously untested.

Notes

This is the over-fetch portion of a broader Bugscache query audit. A separate, layered PR adds a GIN trigram index on bugscache.summary (the substring/similarity search currently can't use the existing btree index) — kept separate so the index change can be measured and reviewed on its own.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.96%. Comparing base (6e680ff) to head (bf76b0f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9554      +/-   ##
==========================================
+ Coverage   82.94%   82.96%   +0.02%     
==========================================
  Files         613      613              
  Lines       35372    35394      +22     
  Branches     3208     3206       -2     
==========================================
+ Hits        29338    29364      +26     
+ Misses       5880     5876       -4     
  Partials      154      154              

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