fix: stop uvicorn threads during shutdown

[Ricky-7-Yan]

What does this PR do?

Fixes #684

On Windows, LitServe runs each Uvicorn server in a thread. The shutdown path treated those threads like multiprocessing workers and called methods that threading.Thread does not provide, leaving the server hanging or logging an attribute error

This change gives Uvicorn worker threads the same small lifecycle surface used by the existing shutdown loop. terminate() requests Uvicorn's graceful exit, while the existing timeout fallback sets force_exit. Process workers keep their current behavior

Validation:

• pytest tests/unit/test_lit_server.py::test_uvicorn_thread_shutdown_signals -q (1 passed)
• ruff check src/litserve/server.py tests/unit/test_lit_server.py
• ruff format --check src/litserve/server.py tests/unit/test_lit_server.py

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? Not needed for this internal lifecycle fix
• Did you write any new necessary tests?

PR review

Anyone in the community is free to review the PR once the tests have passed

Did you have fun?

Yes

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85%. Comparing base (4c9fecc) to head (d79162f).
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Additional details and impacted files

@@         Coverage Diff         @@
##           main   #737   +/-   ##
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- Coverage    85%    85%   -0%     
===================================
  Files        39     39           
  Lines      3285   3293    +8     
===================================
+ Hits       2781   2787    +6     
- Misses      504    506    +2     

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