feat(core): add tRPC protocol assertion

[I-am-Uchenna]

Summary

• add a serialized trpc_ok assertion target for tRPC requests
• detect protocol errors in unbatched, batched, and SuperJSON response envelopes
• surface the tRPC error code and message in assertion results
• expose the new target only in the tRPC assertion editor

Why

tRPC procedures can fail at the protocol level even when an HTTP status check is not the clearest expression of intent. The existing graphql_ok evaluator cannot detect tRPC's singular error object, so a dedicated target is needed.

Successful tRPC assertions continue to evaluate decoded procedure data. Error responses fall back to the raw envelope, where trpc_ok now reports the protocol code and message.

Closes #8.

Validation

• cargo fmt --all --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- --deny warnings