MINOR: Preserve fixed-size binary widths in Arrow schemas - #3803

Rationale for this change

A 16-byte fixed-size Arrow binary field comes back as variable-width Binary after writing a file with the converted Parquet schema. The bytes and nulls survive, but the declared width is lost.

What changes are included in this PR?

Positive fixed-size binary widths now map to fixed-length Parquet storage and back. Zero-width fields keep their BINARY representation because Parquet requires a positive fixed length. Decimal annotations and INT96 mappings are unchanged.

Are these changes tested?

The regression writes a real file containing a 16-byte value and a null row, reads both back, and checks the recovered Arrow schema.

Testing Done

I ran the same regression on the baseline and this change with Java 17 and Maven 3.9.16. From the PR checkout:

git worktree add -b 1fannnw/repro-fixed-binary \
  ../parquet-fixed-binary-before 2df8d02678dab4bb8b926a0d3221cc652984c7ab
git diff 2df8d02678dab4bb8b926a0d3221cc652984c7ab HEAD \
  -- parquet-arrow/pom.xml \
  parquet-arrow/src/test/java/org/apache/parquet/arrow/schema/TestSchemaConverter.java |
  git -C ../parquet-fixed-binary-before apply -

mvn -B -T2 -f ../parquet-fixed-binary-before/pom.xml \
  -pl parquet-arrow -am test \
  '-Dtest=TestSchemaConverter#testFixedBinaryFileRoundTrip' \
  -Dsurefire.failIfNoSpecifiedTests=false

mvn -B -T2 -pl parquet-arrow -am test \
  '-Dtest=TestSchemaConverter#testFixedBinaryFileRoundTrip' \
  -Dsurefire.failIfNoSpecifiedTests=false

stored_type=BINARY
stored_width=0
restored_type=Binary
rows=2, payload_preserved=true, null_preserved=true

expected: Schema<fingerprint: FixedSizeBinary(16)>
 but was: Schema<fingerprint: Binary>

stored_type=FIXED_LEN_BYTE_ARRAY
stored_width=16
restored_type=FixedSizeBinary
rows=2, payload_preserved=true, null_preserved=true

The added dependencies are test-scoped for the file roundtrip.

• Local code review completed

Are there any user-facing changes?

Yes. Positive fixed-size binary fields retain their declared widths when converting schemas in either direction.