fix(adapters): refuse unauthenticated send when capability is absent (FWD-16)

The FWD-08 remediation made every malformed credential fail loudly, but
an authed operation whose registry capability was entirely absent fell
through the build_request match: the request was sent with no credential
and no diagnostic, producing corrupted upstream 401s at call time.

- build_request now returns an INTERNAL error naming the missing
  capability keys (api_key:{ns} / http_token:{ns}) when an auth scheme
  is declared and Capabilities::get is empty; the request is not sent
- auth_scheme: None behavior unchanged (unauthenticated ops stay
  unauthenticated); error message carries key names only, no secret
- module doc: loud-missing matrix now covers malformed name/value AND
  absent capability
- tests: unit loud-error across all three schemes, unchanged-arm pin,
  wire test asserting the upstream receives zero requests (mirrors the
  FWD-08 test family)
- from_openapi/from_jsonschema no_env_vars tests updated: they pinned
  the old silent fall-through; still assert no env material echoes

Verification: cargo test (380 passed), cargo test --all-features
(496 passed), clippy --all-targets -D warnings (default + all-features),
cargo fmt --check — all via scripts/verify.sh

src/adapters/from_jsonschema.rs

@@ -619,7 +619,7 @@ mod tests {
     fn no_env_vars_read_in_build_request() {
         std::env::set_var("OPENAI_API_KEY", "should-not-be-used");
         let ctx = noop_context("req-14", Capabilities::new());
-        let (_, _, _, headers) = build_request(
+        let err = build_request(
             "https://api.openai.com",
             "/v1/chat",
             "POST",
@@ -630,10 +630,17 @@ mod tests {
             &serde_json::json!({"body":{"prompt":"hi"}}),
             &ctx,
         )
-        .unwrap();
+        .expect_err("absent capability is a loud error (FWD-16), never an unauthenticated send");
+        assert_eq!(err.code, "INTERNAL");
         assert!(
-            headers.get(AUTHORIZATION).is_none(),
-            "no auth header when capabilities absent"
+            err.message
+                .contains("capability for namespace `openai` is absent"),
+            "message was: {}",
+            err.message
+        );
+        assert!(
+            !err.message.contains("should-not-be-used"),
+            "error must not echo env material"
         );
         std::env::remove_var("OPENAI_API_KEY");
     }