Fix/default stream singletons

[Monishver11]

Description

closes #1494

This PR ensures that Stream.legacy_default() and Stream.per_thread_default() return singleton instances when called from the base Stream class.

Previously, these methods were returning new Python instances on every call, which deviated from the intended design of exposing default streams as singletons to avoid unnecessary object creation and ensure identity consistency (e.g., s1 is s2). This aligns the implementation with the intended behavior discussed in #1483

Changes

• Implemented logic in legacy_default() and per_thread_default() to return cached cdef stream objects.
• Initialized the internal singletons using the internal _from_handle factory to prevent circular reference issues during Cython module initialization.
• Linked the public-facing module constants LEGACY_DEFAULT_STREAM and PER_THREAD_DEFAULT_STREAM to these same singleton instances.
• Maintained the ability for subclasses of Stream to receive unique instances of the subclass type, ensuring that specialized stream types are not forced into the base singleton.

Testing

(cuda-python-dev) [mc10322@cuda5 cuda_core]$ pytest tests/test_stream.py -v
============================================== test session starts ==============================================
platform linux -- Python 3.10.19, pytest-9.0.2, pluggy-1.6.0 -- /scratch/mc10322/miniconda3/envs/cuda-python-dev/bin/python3.10
cachedir: .pytest_cache
rootdir: /scratch/mc10322/cuda-python/cuda_core
configfile: pytest.ini
collected 33 items

tests/test_stream.py::test_stream_init_disabled PASSED                                                    [  3%]
tests/test_stream.py::test_stream_init_with_options PASSED                                                [  6%]
tests/test_stream.py::test_stream_handle PASSED                                                           [  9%]
tests/test_stream.py::test_stream_is_nonblocking PASSED                                                   [ 12%]
tests/test_stream.py::test_stream_priority PASSED                                                         [ 15%]
tests/test_stream.py::test_stream_sync PASSED                                                             [ 18%]
tests/test_stream.py::test_stream_record PASSED                                                           [ 21%]
tests/test_stream.py::test_stream_wait_event PASSED                                                       [ 24%]
tests/test_stream.py::test_stream_wait_invalid_event PASSED                                               [ 27%]
tests/test_stream.py::test_stream_device PASSED                                                           [ 30%]
tests/test_stream.py::test_stream_context PASSED                                                          [ 33%]
tests/test_stream.py::test_stream_from_foreign_stream PASSED                                              [ 36%]
tests/test_stream.py::test_stream_from_handle PASSED                                                      [ 39%]
tests/test_stream.py::test_legacy_default_stream PASSED                                                   [ 42%]
tests/test_stream.py::test_per_thread_default_stream PASSED                                               [ 45%]
tests/test_stream.py::test_stream_subclassing PASSED                                                      [ 48%]
tests/test_stream.py::test_stream_legacy_default_subclassing PASSED                                       [ 51%]
tests/test_stream.py::test_stream_per_thread_default_subclassing PASSED                                   [ 54%]
tests/test_stream.py::test_stream_legacy_default_public_api PASSED                                        [ 57%]
tests/test_stream.py::test_stream_per_thread_default_public_api PASSED                                    [ 60%]
tests/test_stream.py::test_stream_equality_same_handle PASSED                                             [ 63%]
tests/test_stream.py::test_stream_inequality_different_handles PASSED                                     [ 66%]
tests/test_stream.py::test_stream_equality_reflexive PASSED                                               [ 69%]
tests/test_stream.py::test_stream_equality_symmetric PASSED                                               [ 72%]
tests/test_stream.py::test_stream_type_safety PASSED                                                      [ 75%]
tests/test_stream.py::test_stream_not_equal_operator PASSED                                               [ 78%]
tests/test_stream.py::test_stream_hash_consistency PASSED                                                 [ 81%]
tests/test_stream.py::test_stream_hash_equality_same_handle PASSED                                        [ 84%]
tests/test_stream.py::test_stream_hash_inequality_different_handles PASSED                                [ 87%]
tests/test_stream.py::test_stream_dict_key PASSED                                                         [ 90%]
tests/test_stream.py::test_stream_set_membership PASSED                                                   [ 93%]
tests/test_stream.py::test_builtin_stream_hash PASSED                                                     [ 96%]
tests/test_stream.py::test_default_stream_consistency PASSED                                              [100%]

============================================== 33 passed in 0.38s ===============================================

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[Monishver11]

Hello @leofang. Can you please review this PR for #1494 fix?

[mdboom]

/ok to test ff9b8af

[github-actions]

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