gh-151640: avoid sharing BytesIO buffer in free-threaded builds

[pedramkarimii]

Fixes gh-151640.

In free-threaded builds, BytesIO.read() could return self->buf directly for whole-buffer reads via Py_NewRef(). BytesIO.getvalue() could also return the internal buffer directly. A concurrent writer may then resize the same internal bytes object while another thread decrefs the returned reference, producing a TSAN-reported race between _PyBytes_Resize() and _Py_DecRefShared().

This change avoids exposing the internal BytesIO buffer in Py_GIL_DISABLED builds. Whole-buffer read() and getvalue() now return a copy in free-threaded builds, while keeping the existing fast path unchanged for regular GIL builds.

A focused free-threading regression test was added for concurrent whole-buffer read()/getvalue() and buffer-resizing writes.

Tests run:

• ./python -m test test_free_threading.test_io -v -m test_concurrent_whole_buffer_read_and_resize
• ./python -m test test_free_threading.test_io -v
• ./python -m test test_io -v
• ./python -m test test_free_threading -v
• ./python ../gh151640_reproducer.py

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[pedramkarimii]

Additional context:

This PR is intended to fix the specific _io.BytesIO free-threaded race reported in gh-151640. The issue report focuses on the whole-buffer BytesIO.read() fast path returning self->buf directly via Py_NewRef(), while a concurrent writer may later resize that same internal bytes object.

While investigating the code path, I also found that BytesIO.getvalue() can expose the same internal buffer in a similar way. For that reason, this PR handles both whole-buffer read() and getvalue() in Py_GIL_DISABLED builds.

The chosen fix is intentionally conservative: in free-threaded builds, avoid returning the internal BytesIO buffer directly to user code, while keeping the existing fast path unchanged for regular GIL builds. This avoids changing behavior/performance for non-free-threaded builds and keeps the patch localized to _io.BytesIO.

I also considered fixing this lower in the resize/shared-buffer ownership path, but that seemed more invasive because it would require changing the interaction between SHARED_BUF(), _PyBytes_Resize(), and buffer ownership/copy-on-write behavior. If maintainers prefer preserving this optimization in free-threaded builds, I’m happy to rework the patch in that direction.

[ZeroIntensity]

FT_ATOMIC calls inside a block where Py_GIL_DISABLED is never defined are useless. Let's remove them.