fix: use writeFixed instead of writeBytes for FixedByteBufferWriter

[waterWang]

Bug fix: FixedByteBufferWriter uses writeBytes instead of writeFixed for Avro fixed[N] fields

Fixes #17501

Problem

ValueWriters.FixedByteBufferWriter.write(ByteBuffer, Encoder) calls encoder.writeBytes(bytes) instead of encoder.writeFixed(bytes). For Avro fixed[N] fields this is wrong: writeBytes prepends a zigzag-encoded length prefix before the data, but the Avro reader consumes exactly N bytes for a fixed field and has no length prefix to skip. The stray prefix byte spills into the next field in the record.

Fix

Convert the ByteBuffer to a byte[] and call encoder.writeFixed(arr) instead.

Testing

Existing tests for fixed[N] fields with ByteBuffer values should now pass without corruption. The sibling class FixedWriter (which handles byte[]) already uses writeFixed correctly.

[uros-b]

@waterWang Please note that this may have already been covered in #17502. cc @DuanRuixiao