feat(adapters): enforce advertised input schemas at call time (OAI-18, option a)

Decision: advertise == enforce. The key allowlist (OAI-02) stays as the
first gate with its established unknown-key message; a compiled leaf
validator now runs second, so required/type/enum/pattern/bounds
violations surface as INVALID_INPUT 422 naming the keyword — not as
upstream round-trips.

- new src/adapters/input_validation.rs: CompiledInputSchema compiles an
  op's input_schema once at import with the jsonschema crate (same
  2020-12 dialect publish_schema uses) and validates peer input at call
  time; the compile-time copy is hardened closed-by-default
  (additionalProperties: false injected when absent) so the validator
  reproduces the allowlist's unknown-key semantics; explicit
  additionalProperties:true catch-all and schema values are preserved;
  the original spec value is never mutated
- from_openapi/from_jsonschema import(): compile per registration,
  capture the validator in the handler closure (re-import recompiles —
  the closure capture is the invalidation story); a non-compilable
  input schema fails import loudly (AdapterError::SchemaParse naming
  the operation), matching the publish_schema fail-closed precedent
- from_openapi generated input schemas now carry
  additionalProperties:false explicitly, so the /schema advert states
  the enforced rule and external schema-driven validators reach the
  same verdicts
- forward/forward_stream/build_request gain an
  Option<&CompiledInputSchema> parameter; enforcement runs after the
  allowlist
- round-trip test (review 002 Test-gap 10): the /schema-exported
  input_schema is compiled with the same validator and driven against
  build_request over a 10-input violation matrix — accept-sets exactly
  equal in both directions; the chain-test that lets advertise/enforce
  drift surface as a CI failure
- ADR-066: new decision section (advertise==enforce) with the trust-
  boundary reasoning and the rejected option (b) rationale
- module + enforce_input_schema docs updated to the two-gate shape

cargo test --all-features 596 pass; clippy --all-features/-D warnings,
fmt, doc gates clean.

docs(tasks): mark review-002-fu-oai18-decision done

src/adapters/mod.rs

@@ -6,6 +6,7 @@
 pub mod forward;
 pub mod from_jsonschema;
 pub mod from_openapi;
+pub mod input_validation;
 pub mod openapi_spec;
 pub mod to_openapi;