feat(adapters): from_openapi adapter (parse + forwarding handlers)

Ported FromOpenAPI on the pre-staged foundations:
- openapi_spec.rs (from_json/from_yaml/from_str JSON-first per ADR-051,
  $ref resolution) — already shared with to_openapi
- forward.rs shared forwarding core (build_request/forward/
  forward_stream/parse_sse_frames) — already shared with from_jsonschema

New in this port:
- FromOpenAPI adapter: op-id normalization (declared or
  {method}_{path}), op-type detection (GET->Query, else Mutation;
  200/201 text/event-stream -> Sub), input schema from parameters +
  requestBody 'body', error schemas as HTTP_<status> (ADR-023),
  Internal visibility + FromOpenAPI provenance (ADR-015/022),
  Query/Mutation -> Once, Sub -> Stream (Pub never produced, v1)
- 47 in-module tests: wire-level integration over real TCP
  (echo + capturing servers), bearer/api-key/basic credential
  injection from Capabilities (ADR-014 no-env-vars), SSE streaming,
  YAML + from_str + ADR-051 yes-string guards
- removed dead_code allows from openapi_spec.rs (now consumed)

Verified: cargo test (182 lib), test --all-features (182+10 WS),
clippy -D warnings (both), fmt.

src/adapters/mod.rs

@@ -5,10 +5,12 @@
 
 pub mod forward;
 pub mod from_jsonschema;
+pub mod from_openapi;
 pub mod openapi_spec;
 pub mod to_openapi;
 
 pub use forward::{HttpAuthScheme, HttpServiceConfig};
 pub use from_jsonschema::FromJsonSchema;
+pub use from_openapi::FromOpenAPI;
 pub use openapi_spec::OpenAPISpec;
 pub use to_openapi::to_openapi;

src/adapters/openapi_spec.rs

@@ -181,7 +181,6 @@ impl OpenAPISpec {
         })
     }
 
-    #[allow(dead_code, reason = "consumed by the from_openapi port (next task)")]
     pub(crate) fn resolve_ref(&self, reference: &str) -> Result<Value, AdapterError> {
         if !reference.starts_with("#/") {
             return Err(AdapterError::SchemaParse {
@@ -197,7 +196,6 @@ impl OpenAPISpec {
         Ok(current.clone())
     }
 
-    #[allow(dead_code, reason = "consumed by the from_openapi port (next task)")]
     pub(crate) fn resolve_refs_recursive(&self, schema: &Value) -> Result<Value, AdapterError> {
         match schema {
             Value::Object(obj) => {