Fix comm-central error-line cache selection in get_error_summary

[camd]

Summary

get_error_summary() selects a per-tree error-line cache with:

if job.repository == "comm-central":
    lcache = MemDBCache("cc_error_lines")
else:
    lcache = MemDBCache("mc_error_lines")

But job.repository is a Repository model instance, not a string, so job.repository == "comm-central" is always False. As a result comm-central jobs never used the cc_error_lines cache — they fell through to mc_error_lines and shared a namespace with mozilla-central/autoland error-line counts.

Those counts feed the recent-failure counter and the "new in revision" detection in bug_suggestions_line (which correctly uses str(project.name)), so cross-contaminating the namespaces skews which failure lines get flagged as new for comm-central.

The fix compares job.repository.name, consistent with how bug_suggestions_line already inspects the repository name.

Testing

• Added parametrized unit tests asserting the error-line cache keyroot is chosen from the repository name (comm-central → cc_error_lines; autoland/mozilla-central/try → mc_error_lines). Verified the comm-central case fails against the pre-fix code and passes after.
• tests/model/test_error_summary.py passes (29 tests).

Note

There is a second, currently commented-out occurrence of the same pattern in treeherder/webapp/api/note.py (disabled under Bug 2026428 for performance). It is left untouched here since it is inactive, but would need the same .name fix if ever re-enabled.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.60%. Comparing base (740daac) to head (6ab20ba).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9670      +/-   ##
==========================================
+ Coverage   82.59%   82.60%   +0.01%     
==========================================
  Files         622      622              
  Lines       36580    36596      +16     
  Branches     3279     3340      +61     
==========================================
+ Hits        30213    30230      +17     
+ Misses       6217     5983     -234     
- Partials      150      383     +233     

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