fix: write manifest _FILE as an Avro record

[shyjsarah]

Summary

Follow up on #742 by aligning the Avro type of ManifestEntry._FILE with Java Paimon.

Java's ManifestAvroReader requires _FILE to be a RECORD, but paimon-rust currently writes it as a nullable UNION (["null", record]). This causes Java to reject Rust-written manifests with:

Unexpected Manifest Avro type for field _FILE: expected RECORD but found UNION.

Changes

• Write _FILE directly as a non-null Avro record.
• Add a schema regression test that verifies _FILE is a non-null record.
• Keep a compatibility test proving paimon-rust can still read manifests written with the previous Rust field order and nullable _FILE union.

Testing

• cargo fmt --all -- --check
• cargo test -p paimon --lib (2380 passed, 2 ignored)
• cargo clippy -p paimon --all-targets -- -D warnings

Notes

• No public API changes.
• Existing Rust-written manifests remain readable by paimon-rust.
• The change only affects the schema used for newly written manifest files.

[QuakeWang]

LGTM