[Python][Parquet] Comparing metadata or schemas with None can crash Python #53500

Describe the bug

Parquet metadata and schema equality can terminate the Python interpreter when the other operand is None. For example, checking metadata.row_group(0).column(0).statistics == None causes SIGSEGV instead of returning False.

The equals() methods accept a typed Cython object without rejecting None, then access its native pointer. FileMetaData.equals() already uses other not None; the other five metadata/schema classes do not.

Reproducer

Run in an isolated Python process:

import pyarrow as pa
import pyarrow.parquet as pq

sink = pa.BufferOutputStream()
pq.write_table(pa.table({"value": [1, 2, 3]}), sink)
metadata = pq.read_metadata(pa.BufferReader(sink.getvalue()))
statistics = metadata.row_group(0).column(0).statistics
print(statistics == None)  # process exits with SIGSEGV

The same result occurs with .equals(None), row-group metadata, column-chunk metadata, and column schemas. On Arch Linux / CPython 3.14.8, ParquetSchema comparison also crashes. On Ubuntu / CPython 3.12.14 it happens to return False. This was reproduced with PyArrow 26.0.0 and with the current main _parquet extension built from source at 9b34c4a6a8856212b885f6a1e934795af04f3192 against the matching 26.0.0 SDK.

Expected: normal equality with None returns False; explicit .equals(None) rejects the invalid argument with TypeError, consistently with FileMetaData.

Existing work checked

Open PRs #51161 and #51163 change other Parquet entry points (constructors, sorting, binary types). Their current patches do not change these five equals() methods. Searches for metadata/schema/statistics comparison and None-related open PRs found no overlapping fix.

Component(s)

Python, Parquet