fix: ValueWriters.FixedByteBufferWriter should use writeFixed(), not writeBytes() [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Description

For Avro fixed[N] fields, the encoder must write fixed-length data
without a length prefix. writeBytes() writes a length-prefixed bytes
value, which corrupts the Avro binary encoding for fixed[N] schema.

Fix

Replace encoder.writeBytes(bytes) with the correct fixed-length write:

byte[] arr = new byte[length];
bytes.duplicate().get(arr);
encoder.writeFixed(arr, 0, length);

Testing

• The Preconditions.checkArgument validates bytes.remaining() == length before the write
• duplicate() creates a view so the original buffer's position is not modified
• All existing tests should pass (the fix only changes the Avro encoding method, not the data)

Fixes #17501

[uros-b]

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