GH-48254: [Python][Parquet] Support extension types in read_schema

[Kuinox]

Rationale for this change

pq.read_schema drops extension types (UUID comes back as fixed_size_binary[16]), while ParquetFile.schema_arrow and read_table preserve them. Schema inspection via metadata should match table/extension behavior.

What changes are included in this PR?

• Plumb arrow_extensions_enabled into read_schema and return schema_arrow when enabled so extension types are preserved.
• Add regression test ensuring UUID extension types are retained by read_schema and downgraded to binary(16) when extensions are disabled.

Are these changes tested?

• Yes: added unit test test_read_schema_uuid_extension_type

Are there any user-facing changes?

• Behavior improvement: read_schema now preserves extension types (e.g., UUID) when extensions are enabled; no API break

Notes:

• I don't know if the fact the column types being returned are now extension<arrow.uuid> instead of fixed_size_binary[16], is considered a breaking change.
• This PR patch was AI generated, but I personally reviewed it, the scope is small, and it looks fine to me.

• GitHub Issue: [Python][Parquet] read_schema drops extension types (UUID returned as fixed_size_binary[16]) #48254

[github-actions]

⚠️ GitHub issue #48254 has been automatically assigned in GitHub to PR creator.