Decimals in JSON subscriptions of Avro topics are invalid

[moscicky]

Test that reproduces the bug: b9773ce

org.opentest4j.AssertionFailedError: 
expected: "1.20"
 but was: "x"

[MateuszDobrowolski]

Can i take care of it? : )

[szczygiel-m]

Hi, sure 😄 in case of any questions/problems feel free to ask

[MateuszDobrowolski]

@szczygiel-m I have prepared the proposition of the fix. Could I ask for permission to create a draft PR?

[MateuszDobrowolski]

Proposal: During AVRO-to-JSON conversion, we transform messages with the logical decimal type into a BigDecimal and then into a String. This ensures that subscription consumers receive the decimal value itself rather than the byte buffer contents. This conversion does not apply to AVRO-to-JSON conversion for previews of messages published to the topic.
#1920