[Bug] Avro throws ClassCastException when writing MULTISET with a non-string element type

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Paimon version
master @ a5e87eb (2.1-SNAPSHOT)

Compute Engine
Engine-agnostic — the failure is in paimon-format (avro).

Minimal reproduce step
Create a table with a MULTISET<INT> column and 'file.format' = 'avro', then insert one row.

DDL succeeds: AvroFileFormat.validateDataFields() only calls AvroSchemaConverter.convertToSchema(), which handles MULTISET and MAP in one branch and emits the array-of-record encoding for a non-string key.

The first write then fails:

java.lang.ClassCastException: MultisetType cannot be cast to ArrayType
  at org.apache.paimon.format.avro.AvroSchemaVisitor.visit(AvroSchemaVisitor.java:61)
  at org.apache.paimon.format.avro.FieldWriterFactory.visitUnion(FieldWriterFactory.java:107)

AvroSchemaVisitor.visit() handles only MapType and VectorType in its ARRAY branch and casts anything else to ArrayType, which is final. Reads go through the same method.

What doesn't meet your expectations?
Expected: the row is written and read back, as ORC and Parquet already do for the same schema.
Actual: every write of a MULTISET with a non-string element type throws.

Anything else?
MULTISET<STRING> is unaffected — it uses the native avro map encoding.

Are you willing to submit a PR?

• I'm willing to submit a PR!