fix: disallow requests to /inserts-stream if insert values disabled

[vcrfxia]

Description

Fixes #5590. This will also need to be backported to 5.5.x.

Testing done

Not sure what makes sense in terms of automated testing. Endpoint-specific code currently can only be tested with integration tests, not unit tests. Because this functionality requires different server configs, we'd have to add a separate integration test file just for this, which feels really heavy-weight. @purplefox do you have recommendations?

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[vcrfxia]

Turns out this doesn't need to be backported to 5.5.x since the /inserts-stream functionality wasn't implemented on 5.5.x:

ksql/ksqldb-api/src/main/java/io/confluent/ksql/api/plugin/KsqlServerEndpoints.java

Line 155 in 6c0e078

return null;