fix(ci): handle sentinel input in request_queue gracefully

[aniketmaurya]

What does this PR do?

Devs won't see a lot of error msgs in the CI.

We exit the background process loop by sending (None, None, None) to the queue. We don't handle this gracefully and even though the loop is exited, we end up printing a lot of errors in the CI. This PR, will gracefully handle the termination for clearer CI.

Follow up

• Clean the rest of the test errors

Changes

• Added a new exception class _StopLoopError to handle sentinel values in request processing loops.
• Updated collate_requests and loop classes to raise _StopLoopError when a sentinel value is encountered, allowing for clean exit from loops.
• Refactored request handling in SingleLoop and StreamingLoop to improve clarity and maintainability.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Attention: Patch coverage is 90.24390% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85%. Comparing base (17f8ee0) to head (c0c7867).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #589   +/-   ##
===================================
- Coverage    85%    85%   -0%     
===================================
  Files        39     39           
  Lines      3017   3049   +32     
===================================
+ Hits       2564   2588   +24     
- Misses      453    461    +8     

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[Copilot]

Pull Request Overview

This PR introduces graceful handling of sentinel values in request queue processing to reduce failure messages in CI. It implements a new exception mechanism for cleanly exiting processing loops when shutdown signals are received.

• Added _StopLoopError exception class for handling sentinel value shutdown signals
• Updated all request processing loops to check for sentinel values and exit gracefully
• Refactored request queue handling across streaming, single, and batched loops for consistency

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/litserve/loops/base.py	Added _StopLoopError exception and sentinel handling in collate_requests function
src/litserve/loops/simple_loops.py	Updated single, async, and batched loops to handle sentinel values gracefully
src/litserve/loops/streaming_loops.py	Added sentinel value detection and graceful exit in streaming loop
tests/unit/test_batch.py	Added test coverage for sentinel value handling in collate_requests

Comments suppressed due to low confidence (1)

src/litserve/loops/simple_loops.py:301

• The method call has been changed from collate_requests to self.get_batch_requests but the import for collate_requests was removed. This suggests get_batch_requests is being used instead, but this change is inconsistent with the other loops which still use collate_requests directly.

                batches, timed_out_uids = self.get_batch_requests(