fix: Add decimal logical type handling in ToAvroDataConverter

Added case for 'decimal' logical type in convertFieldValue method to properly
convert java.math.BigDecimal values to ByteBuffer format expected by Avro.

This fixes the failing AvroFormatWriterTest 'should write decimal data from
the header' test.

(cherry-picked from c6de95d)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: andmarios

kafka-connect-cloud-common/src/main/scala/io/lenses/streamreactor/connect/cloud/common/sink/conversion/ToAvroDataConverter.scala

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2025 Lenses.io Ltd
+ * Copyright 2017-2026 Lenses.io Ltd
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -145,6 +145,12 @@ object ToAvroDataConverter {
             case d: Date => d.getTime * 1000L
             case other => other
           }
+        case Some("decimal") =>
+          value match {
+            case bd: java.math.BigDecimal =>
+              ByteBuffer.wrap(bd.unscaledValue().toByteArray)
+            case other => other
+          }
         case _ =>
           // No logical type or unhandled logical type - convert based on physical schema type
           convertBySchemaType(value, targetSchema)