adopt: validate DSL output against the engine's imposter JSON Schema (sdk-conformance schema/imposter.schema.json) - #314

rift 0.22.0 ships the engine's imposter grammar as a JSON Schema (draft 2020-12) inside the sdk-conformance tarball, at schema/imposter.schema.json (rift#1342). Every object in it is closed (additionalProperties: false) except the provider maps. A document that validates therefore carries no key the engine never reads.

Changes

• New SchemaConformanceTest, the conformance module's "Schema" gate:
  • dslOutputValidates: every DSL-built fixture validates against the schema.
  • codecOutputValidates: every fixture's fromJson → toJson output validates. A parse failure outside KnownFidelityGaps is rethrown.
  • modeledKeysAreKeysTheEngineReads: each of the 7 model types' MODELED_KEYS must be a subset of its $defs properties. The keys are read reflectively.
  • predicateParametersAreExactlyTheSchemasNonOperatorKeys: checks PredicateOperation.RESERVED_KEYS in both directions.
  • theValidatorRejectsAKeyTheEngineNeverReads: negative cases (an unread stub key, an unread _rift key reached through $ref, a mistyped statusCode), so a no-op validator cannot pass.
• Corpus.schema(): finds the schema by file presence. A corpus without it skips the gate. The RIFT_IT=1 lane fails instead when a 0.22.0+ corpus lacks it.
• Test-scope dependency com.networknt:json-schema-validator 1.5.6. It lives in the conformance module, which is not deployed, so core stays dependency-free. Jackson is aligned by the root BOM.
• The conformance README documents the gate.

The issue asked to "derive MODELED_KEYS from $defs". Core doesn't ship the schema, so deriving the keys at runtime isn't possible. Instead, the hand-kept lists are checked against the schema on every conformance run.

Verification

• ./mvnw -B verify -Drift.corpus.root=<0.22.0 corpus>: green. SchemaConformanceTest ran 33 tests (2 aborted: fixtures with no DSL builder, or a known fidelity gap).
• Without a corpus, all 3 test containers skip.
• Mutation checks, each reverted afterwards:
  • re-adding backend to RiftFlowStateConfig.MODELED_KEYS → modeledKeysAreKeysTheEngineReads fails with {RiftFlowStateConfig=[backend]};
  • making Stub emit an unread key → 29 failures, each naming property 'nmae' is not defined in the schema.

Closes #300

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