BE: fixes: avro schema ser for nullable enums

[DimaVilda]

• Breaking change?

What changes did you make?
Fixed incorrect JSON schema generation for nullable enum types in Avro schemas. Previously, nullable enums were generated with generic "enum" key instead of the actual enum name, causing incorrect serialization format with error:

400 Bad Request
json value '{"enum":"CLUBS"}' is cannot be converted to any of union types

See bug reproduction in attached video!

Is there anything you'd like reviewers to focus on?
We can think about changing nullable enums json generation. I.e. instead of :

      {
        "enum_nullable_union": { "Suit": "SPADES"}
      }

Simply use

      {
        "enum_nullable_union": "SPADES"
      }

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• No need to
• Manually (please, describe, if necessary)
• [ x] Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

kafkaUiBug.mp4

[github-actions]

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[DimaVilda]

hey there!
I'd like to suggest you change Avro ser process for union nullable enums so it looks like simple enum in json.
Example: consider you have a following nullable enum in your Avro *.avdl schema:

"name": "booksize"
"type" : [ "null", {
"type" : "enum",
"name" : "BookSizeEnum",
"symbols" : [ "ORIGINAL", "A3", "A4" ]
} ]

then kafka-ui createUnionSchema AFTER MY FIX OFC method will transform it to:

"booksize": {
"oneOf": [
{ "type": "null" },
{
"type": "object",
"properties": {
"BookSizeEnum": { // NOTE that now you have "enum" in master
"type": "string",
"enum": ["ORIGINAL", "A3", "A4"]
}
}
}
]
}

Which will generate a message for topic like:

{
"booksize": { "BookSizeEnum": "ORIGINAL"}
}

However a simple enum (not nullable) looks like:

{
"BookSizeEnum": "ORIGINAL"
}
In my opinion it make more hustle and mess. What I suggest to do is simplify it and make look a nullable enum like a simple not union nullable enum so for:

"name": "booksize"
"type" : [ "null", {
"type" : "enum",
"name" : "BookSizeEnum",
"symbols" : [ "ORIGINAL", "A3", "A4" ]
} ]

Kafka-ui will generate:

{
"BookSizeEnum": "ORIGINAL"
}

or for null:

{
"BookSizeEnum": null
}

What do you think? Can I create a following issue for it and then proceed with this ?