refactor(gateway): delete dead accessors and FromRef impls (review 002 COV-13)

Coverage-confirmed dead code (every binary, zero hits):

- server/state.rs: drop FromRef<RouterState> impls for
  Arc<OperationRegistry> and Arc<dyn IdentityProvider> — no route
  extracts these types; the auth middleware receives the provider
  directly via from_fn_with_state
- gateway/dispatch.rs: drop identity_provider() and resolve_bearer()
  accessors; resolve_bearer's doc promised an auth hook the middleware
  never calls (spec/code drift). Wire-or-delete resolved to delete:
  bearer resolution lives in the middleware (SRV-11 single-resolve
  ordering), the dispatch spine only needs the per-call
  Option<Identity>. GatewayDispatch::new consequently takes the
  registry alone (GatewayState loses its unused identity_provider
  passthrough; dispatch.rs/to_mcp.rs tests simplified)
- websocket/upgrade.rs: drop FromRef<SessionState> for
  Arc<OperationRegistry> — no router carries SessionState as its state
  type; the inverse FromRef<Arc<OperationRegistry>> for SessionState
  (custom upgrade routes, integration tests) remains

Verification: ./scripts/verify.sh (352 passed), ./scripts/verify.sh
--all-features (466 passed), clippy -D warnings, fmt --check.

src/websocket/upgrade.rs

@@ -141,12 +141,6 @@ impl SessionState {
     }
 }
 
-impl axum::extract::FromRef<SessionState> for Arc<OperationRegistry> {
-    fn from_ref(state: &SessionState) -> Self {
-        Arc::clone(&state.registry)
-    }
-}
-
 /// `FromRef` chain: a bare `Arc<OperationRegistry>` router state lifts
 /// into the handler's [`SessionState`]; a full `RouterState` carries
 /// the shared [`WsSessions`] instance and lifts through its own impl.