Fix bug in draining non-factory-queued routing

[slawlor]

When the queueing occurs on workers directly, the factory wasn't tracking the in-flight message count correctly, such that the worker might still have work in-flight when requesting a draining operation.

This change fixes 2 things

1. The evaluation of if a worker is "empty" of all jobs, i.e. drained, by checking the current & queued jobs
2. The tracking of the current job on the worker, when a worker has locally dequeued the next job.

Test coverage added for this scenario. Prior to the fix, the added test fails with

thread 'factory::tests::draining_requests::test_worker_drains_when_worker_queueing' panicked at ractor/src/factory/tests/draining_requests.rs:221:5:
assertion `left == right` failed
  left: 999
 right: 1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

since the factory was marked "drained" prior to all messages completing. Now it passes with this patch.

Also docs updated.

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 80.99%. Comparing base (642bdcd) to head (ae4c55e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ractor/src/factory/tests/draining_requests.rs	91.48%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #353      +/-   ##
==========================================
+ Coverage   80.95%   80.99%   +0.03%     
==========================================
  Files          70       70              
  Lines       14367    14415      +48     
==========================================
+ Hits        11631    11675      +44     
- Misses       2736     2740       +4     

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