fix(adapters): reject unbound placeholders when from_jsonschema input_schema has no properties

The eager placeholder-binding check in spec_name_references_undeclared
short-circuited on an empty properties set, letting
FromJsonSchema::new accept a template whose placeholder could never be
bound (every later call failed INTERNAL, or INVALID_INPUT if a peer
supplied the key). Drop the early return so any placeholder against a
declared-nothing (or properties-less) input_schema is unbound, matching
the from_openapi unbound_placeholders path.

- add construction-time test: no-properties schema + {id} template
  fails with the placeholder error; no-properties schema + placeholder-
  free template still constructs and imports as Visibility::Internal

Verification: scripts/verify.sh (342 passed), scripts/verify.sh
--all-features (454 passed); clippy -D warnings and fmt --check clean

src/adapters/from_jsonschema.rs

@@ -170,9 +170,6 @@ fn spec_name_references_undeclared(spec: &OperationSpec, path_template: &str) ->
         .and_then(|p| p.as_object())
         .map(|p| p.keys().cloned().collect::<Vec<_>>())
         .unwrap_or_default();
-    if properties.is_empty() {
-        return false;
-    }
     let mut rest = path_template;
     while let Some(start) = rest.find('{') {
         let Some(end_rel) = rest[start..].find('}') else {
@@ -722,4 +719,56 @@ mod tests {
             Err(e) => panic!("expected SchemaParse, got {e}"),
         }
     }
+
+    #[test]
+    fn construction_rejects_placeholder_when_input_schema_has_no_properties() {
+        let spec = OperationSpec::new(
+            "svc/getWidget",
+            OperationType::Query,
+            Visibility::Internal,
+            serde_json::json!({"type":"object"}),
+            serde_json::json!({"type":"object"}),
+            vec![],
+            AccessControl::default(),
+            None,
+        );
+        let result = FromJsonSchema::new(
+            spec,
+            test_config("svc", "https://api.example.com"),
+            "/widgets/{id}".to_string(),
+            "GET".to_string(),
+            test_http_client(),
+        );
+        match result {
+            Err(AdapterError::SchemaParse { message }) => {
+                assert!(message.contains("placeholder"), "message was: {message}");
+                assert!(message.contains("{id}"), "message was: {message}");
+            }
+            Ok(_) => {
+                panic!("no-properties schema with a placeholder template must fail at construction")
+            }
+            Err(e) => panic!("expected SchemaParse, got {e}"),
+        }
+
+        let spec = OperationSpec::new(
+            "svc/listWidgets",
+            OperationType::Query,
+            Visibility::Internal,
+            serde_json::json!({"type":"object"}),
+            serde_json::json!({"type":"object"}),
+            vec![],
+            AccessControl::default(),
+            None,
+        );
+        let adapter = FromJsonSchema::new(
+            spec,
+            test_config("svc", "https://api.example.com"),
+            "/widgets".to_string(),
+            "GET".to_string(),
+            test_http_client(),
+        )
+        .expect("placeholder-free template with a no-properties schema builds");
+        let bundles = futures::executor::block_on(adapter.import()).unwrap();
+        assert_eq!(bundles[0].spec.visibility, Visibility::Internal);
+    }
 }