Handle null Cython close requests

[fallintoplace]

Summary

• make UCXEndpoint.close() return None when C++ returns an empty close request
• guard UCXRequest request-pointer access before dereferencing
• add a low-level regression for calling endpoint close while close is already in progress

Why

Endpoint::closeRequest() can return nullptr when the endpoint is already closing or closed. Wrapping that empty shared_ptr as UCXRequest lets the Cython destructor and accessors dereference self._request.get() even though it is null.

Returning None keeps repeated close calls idempotent for cleanup paths, while the UCXRequest guard prevents a crash if an empty wrapper is ever constructed by another path.

Validation

• git diff --check
• uvx --from 'cython>=3.2.2' cython -3 --cplus python/ucxx/ucxx/_lib/libucxx.pyx -o .cython-check/libucxx.cpp
• uvx ruff check python/ucxx/ucxx/_lib/tests/test_endpoint.py
• Tried cmake -S cpp -B cpp/build/local-check -G Ninja -DBUILD_TESTS=OFF -DBUILD_BENCHMARKS=OFF -DBUILD_EXAMPLES=OFF -DUCXX_ENABLE_CCCL=OFF; configure is blocked locally because UCX is not installed (ucxConfig.cmake / ucx-config.cmake not found).

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