GH-51044: [Python] Reject read-only readinto destinations

[carrerasdarren-cell]

Rationale for this change

BufferReader.readinto() currently segfaults when passed a read-only destination such as bytes or a read-only memoryview. py_buffer() creates an immutable Arrow buffer, mutable_data() returns a null pointer, and the C++ read path attempts to copy into that pointer. A Python API misuse should raise a Python exception rather than terminate the interpreter.

What changes are included in this PR?

• Validate that a NativeFile.readinto() destination is mutable before obtaining its writable pointer.
• Raise TypeError for immutable destinations.
• Add regression coverage for bytes and read-only memoryview destinations.

Are these changes tested?

Yes. The focused readinto tests pass against a locally compiled patched pyarrow.lib. The stock 25.0.1 wheel exits with status 139 for the issue reproducer, while the patched build raises the expected TypeError; the writable bytearray control continues to read successfully.

Are there any user-facing changes?

Yes. Passing a read-only destination to NativeFile.readinto() now raises TypeError instead of terminating the interpreter. Writable-buffer behavior is unchanged.

Fixes #51044.

This contribution was developed with assistance from OpenAI Codex. I reviewed, tested, and take responsibility for the patch and this description.

• GitHub Issue: [Python] BufferReader.readinto() segfaults on a read-only destination #51044