fix: use idle() instead of length() to guard queue drain

[scolladon]

Explain your changes

Race condition fix in DiffLineInterpreter.process(). The async.queue.length() method only counts items waiting in the queue, not items currently being processed by workers. When all items were dequeued but workers were still running, length() returned 0 and drain() was skipped — causing the method to return incomplete results non-deterministically.

Replaced processor.length() > 0 with !processor.idle(), which correctly checks both waiting AND running items.

Note: drain() cannot be called unconditionally because it hangs forever on an already-idle queue in async@3.x. The guard is necessary but must use idle() instead of length().

Does this close any currently open issues?

closes #1235

• Jest tests added to cover the fix.
• NUT tests added to cover the fix.
• E2E tests added to cover the fix.

Any particular element that can be tested locally

Run npx jest __tests__/unit/lib/service/diffLineInterpreter.test.ts — the new test "Given slow handlers, When queue workers finish after enqueuing, Then all results are collected" verifies that delayed handler results are fully collected.

Any other comments

Root cause analysis:

Queue state	length()	running()	idle()	Old guard	Correct behavior
No items pushed	0	0	true	skip drain ✓	skip drain
Items queued, not started	>0	0	false	await drain ✓	await drain
All dequeued, still running	0	>0	false	skip drain ✗	await drain
All done	0	0	true	skip drain ✓	skip drain

Controlled test showed 100% failure with the old guard vs 0% failure with !idle().

[github-actions]

Published under dev-1236 npm channel.

$ sf plugins install sf-git-merge-driver@dev-1236

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c6c20ab) to head (26471f8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1236   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           58        58           
  Lines         1544      1544           
  Branches       194       194           
=========================================
  Hits          1544      1544           

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[github-actions]

Shipped in release v6.33.0.
Version v6.33.0 will be assigned to the latest npm channel soon
Install it using either v6.33.0 or the latest-rc npm channel

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