test(unit): split queue.test.ts and backfill.test.ts into smaller files

[JSONbored]

Summary

• test/unit/queue.test.ts (33,461 lines / 810 tests) and test/unit/backfill.test.ts (6,874 lines / 236 tests) each ran as one atomic unit under a single vitest worker. In a real CI run, queue.test.ts alone accounted for ~282s (~half the wall-clock) of the full coverage run's ~565s duration, since vitest schedules whole test files to one worker and can't parallelize within a file.
• Split each into several files along verified-safe boundaries: no shared mutable state crosses a file boundary, and hoisted helper functions used across sections were promoted into each file's shared header (found via a TypeScript-AST structural pass, not indentation grepping, after an initial indentation-based attempt silently mis-attributed a describe block's real end line).
• queue.test.ts -> queue.test.ts, queue-2.test.ts, queue-3.test.ts, queue-4.test.ts, queue-5.test.ts, queue-lifecycle-guards.test.ts.
• backfill.test.ts -> backfill.test.ts, backfill-2.test.ts.
• Fixes two stale comments in src/queue/processors.ts that named the old monolithic queue.test.ts for a test that moved.

Why

Pairs with #4815 (CI coverage sharding): splitting these two files is what lets the new matrix shards actually balance -- without it, one shard would always be stuck holding a ~5-minute file no matter how many shards exist.

Test plan

• npx tsc --noEmit clean
• npx vitest run on the old file(s) vs. all new split files: identical test counts and all passing (810/810 for queue*, 236/236 for backfill*)
• Full local suite (npx vitest run --coverage) green
• Confirmed no cross-file references were missed by grepping every hoisted helper's full usage range against the pristine originals before cutting

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.23%. Comparing base (62d91f8) to head (56ae5ab).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4814   +/-   ##
=======================================
  Coverage   94.23%   94.23%           
=======================================
  Files         442      442           
  Lines       38845    38845           
  Branches    14150    14150           
=======================================
  Hits        36607    36607           
  Misses       1577     1577           
  Partials      661      661           

Files with missing lines	Coverage Δ
src/queue/processors.ts	95.67% <ø> (ø)

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.