Add CI check that auto-generated code is up to date.

[StephenButtolph]

#1472 was incorrectly merged (without regenerating the pb files). Currently we use auto generated code for our test mocks (mockgen) and for our proto (buf) message definitions.

We should add a CI check (perhaps 2 separate workflows) that ensures that

• running ./scripts/protobuf_codegen.sh does not change any files.
• running ./scripts/mock.gen.sh does not change any files.

[github-actions]

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