GH-50831: [Python] Avoid aborting on read-only output buffers

[erichanwang]

Rationale for this change

pa.output_stream() currently aborts the Python process when passed a read-only buffer such as pa.py_buffer(b'x') or a memoryview, because the C++ fixed-size buffer writer checks buffer->is_mutable(). A Python API misuse should raise a Python exception instead of terminating the interpreter.

What changes are included in this PR?

• python/pyarrow/io.pxi: validate that a buffer passed to FixedSizeBufferWriter is mutable and raise a Python ValueError before constructing the C++ writer.
• python/pyarrow/tests/test_io.py: add regression coverage for read-only buffers and preserve successful writes to mutable buffers.

Fixes #50831.

Are these changes tested?

Yes. Focused pyarrow IO regression tests were added for the read-only buffer path and the valid mutable-buffer path. The local Arrow checkout also passes git diff --check; the available Arrow CI checks are passing.

Are there any user-facing changes?

Yes. Passing a read-only buffer to pa.output_stream() now raises a Python exception instead of aborting the process. Valid mutable-buffer behavior is unchanged.

This PR contains a "Critical Fix". This fixes a bug that causes a crash when the API contract is upheld by passing a buffer object that is accepted by the Python API but is read-only.

• GitHub Issue: [Python] pa.output_stream() aborts the process on a read-only buffer #50831

[github-actions]

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[Reranko05]

Can you follow the Arrow PR desc template and edit the PR desc accordingly.