GH-51229: [Python] Raise instead of crashing on unopened resize

[1fanwang]

Rationale for this change

pa.MemoryMappedFile() can be constructed without opening a file. Calling resize on that object dereferences an empty native handle and takes down the Python process with a SIGSEGV, so an application cannot catch or recover from it.

What changes are included in this PR?

The resize path now applies the same open-state check the other file operations already use, before it reaches the native resize call. A directly constructed object raises ValueError("I/O operation on closed file").

Are these changes tested?

Yes. The regression drives the call in a subprocess so a crash is observable rather than fatal to the test run.

$ git checkout main -- python/pyarrow/io.pxi && python -m pytest \
    python/pyarrow/tests/test_io.py -k memory_map_resize -q
E  subprocess.CalledProcessError: Command '[...]' died with <Signals.SIGSEGV: 11>.
1 failed, 162 deselected in 0.73s

$ git checkout HEAD -- python/pyarrow/io.pxi && python -m pytest \
    python/pyarrow/tests/test_io.py -k memory_map_resize -q
2 passed, 161 deselected in 0.23s

Are there any user-facing changes?

Yes. Misuse of a directly constructed memory-mapped file now raises a Python exception instead of terminating the process.

This PR contains a "Critical Fix". It fixes a process crash reachable from ordinary Python-level object state.

• GitHub Issue: [Python] MemoryMappedFile().resize() segfaults when no file has been opened #51229

[github-actions]

⚠️ GitHub issue #51229 has been automatically assigned in GitHub to PR creator.

[Copilot]

🟢 Approval recommended

The change correctly prevents a reproducible segfault by aligning resize() with existing open-state checks, and the regression is covered by a subprocess-based test.

Pull request overview

This PR addresses a Python-level crash in pyarrow.MemoryMappedFile.resize() when MemoryMappedFile() is constructed without opening a file, by adding an open-state guard so the misuse raises a catchable exception instead of segfaulting.

Changes:

• Add _assert_open() to the MemoryMappedFile.resize() wrapper before calling the native resize implementation.
• Add a regression test that exercises the behavior in a subprocess so a segfault would be observable without killing the pytest runner.

File summaries

File	Description
python/pyarrow/io.pxi	Adds an open-state assertion to MemoryMappedFile.resize() to prevent dereferencing a null native handle.
python/pyarrow/tests/test_io.py	Adds a subprocess-based regression test ensuring uninitialized MemoryMappedFile().resize() raises ValueError instead of crashing.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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