AVRO-4225: Fix ClassCastException in FastReaderBuilder for java-class attribute

[santosh-d3vpl3x]

What is the purpose of the change

Fixes AVRO-4225: When using GenericDatumReader with schemas containing java-class attributes on string fields (e.g., java.math.BigDecimal), FastReaderBuilder throws:

ClassCastException: Utf8 cannot be cast to String

The bug is in getTransformingStringReader() which casts the result of stringReader.read() directly to String, but GenericData returns Utf8.

The Fix

Explicitly handle both Utf8 and String types, consistent with the rest of the Avro codebase:

Object value = stringReader.read(null, decoder);
String stringValue = value instanceof Utf8 ? ((Utf8) value).toString() : (String) value;
return transformer.apply(stringValue);

Verifying this change

Added test class FastReaderBuilderJavaClassTest with two test cases:

• genericDatumReaderWithJavaClassAttribute - tests union type with java-class
• genericDatumReaderWithDirectJavaClassString - tests direct string field with java-class

Both tests fail without the fix (ClassCastException) and pass with the fix.

Documentation

• Does this pull request introduce a new feature? No
• If yes, how is the feature documented? N/A

[gabru-md]

this will be useful 👍🏻