test: align unit test package names

[sbalabanov]

Summary

• require unit tests to use the package under test in AGENTS.md
• move the consumer and composite validator unit tests into their implementation packages
• replace consumer test dependencies that would create import cycles with local test doubles and topic keys
• regenerate Bazel BUILD metadata

Testing

• make test
• make lint
• make check-tidy
• make check-gazelle

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