Call to undefined method in SchemaRegistryClient.post_new_schema()

[aiven-anton]

What happened?

Discovered when working on static type checking coverage.

On this line there's a call to references.json() where references: Reference, however Reference has no .json() method.

https://github.com/aiven/karapace/blob/42b14c71543b878c42c11025ab266122d61550d3/karapace/serialization.py#L92

I tried adding raise NotImplementedError on that line, and confirmed that there is no test coverage there. Presumably, a case should be added to tests/integration/test_client_protobuf.py.

What did you expect to happen?

• Test suite should be failing.
• Correct method should be called.

[aiven-anton]

cc @libretto