feat(adapters): structural path-placeholder values are an INVALID_INPUT error (FWD-18)

The routing half of FWD-18 was already in place (placeholder keys skip
query routing before value-shape inspection); this pins it with a test
and decides the structural-value outcome: an object/array value under a
placeholder key now fails with INVALID_INPUT instead of splicing the
minified JSON into the path segment. Scalars (string/number/bool/null)
render as before.

src/adapters/forward.rs

@@ -420,11 +420,26 @@ fn scalar_value_to_string(value: &Value) -> String {
 /// Percent-encoding scheme used by [`render_path_template`]: a value
 /// containing `/`, `?`, `#`, or `\` — and any raw `%` it carries — is
 /// encoded so that a rendered value stays one literal path segment
-/// (FWD-01 traversal/ smuggled-segments gate). Path parameters are never
-/// rejected: they are rendered safely instead.
-pub(crate) fn value_to_path_segment(value: &Value) -> String {
-    let raw = scalar_value_to_string(value);
-    utf8_percent_encode(&raw, PATH_VALUE_ENCODE_SET).to_string()
+/// (FWD-01 traversal/ smuggled-segments gate). Scalars are never
+/// rejected: they are rendered safely instead. Object/array values
+/// under a placeholder key are rejected with `INVALID_INPUT` (FWD-18):
+/// a path placeholder designates exactly one literal segment, so a
+/// structural value has no faithful rendering — the pre-decision
+/// behavior spliced the minified JSON into the path *and* emitted the
+/// key as a query parameter (double-routed, neither faithful).
+pub(crate) fn value_to_path_segment(value: &Value) -> Result<String, CallError> {
+    let raw = match value {
+        Value::String(_) | Value::Number(_) | Value::Bool(_) | Value::Null => {
+            scalar_value_to_string(value)
+        }
+        other => {
+            return Err(CallError::invalid_input(format!(
+                "path placeholder value must be a scalar (string, number, boolean, or null); got {}: structural values have no faithful single-segment rendering",
+                type_name_of(other)
+            )))
+        }
+    };
+    Ok(utf8_percent_encode(&raw, PATH_VALUE_ENCODE_SET).to_string())
 }
 
 /// Raw form of a scalar input value for query emission: `&`/`=`
@@ -469,7 +484,7 @@ pub(crate) fn render_path_template(
                 continue;
             }
         };
-        out.push_str(&value_to_path_segment(raw_value));
+        out.push_str(&value_to_path_segment(raw_value)?);
         rest = &tail[end + 1..];
     }
     out.push_str(rest);
@@ -1511,6 +1526,88 @@ mod tests {
         assert!(q.contains("q=rust") && q.contains("debug=true"), "{q}");
     }
 
+    /// FWD-18, routing half pin: a key that matched a placeholder never
+    /// also emits as a query parameter — the skip in `build_request`
+    /// precedes query routing and is value-shape-agnostic. The scalar
+    /// case renders into the path only; the object-value case is
+    /// rejected (see the structural-value test below), so here the
+    /// companion non-placeholder key proves query routing stays intact
+    /// around the placeholder skip.
+    #[test]
+    fn placeholder_keys_never_double_route_as_query_params() {
+        let url = request_url(
+            "https://api.example.com",
+            "/repos/{owner}",
+            json!({"owner": "octocat", "extra": "q-val"}),
+        )
+        .expect("scalar placeholder builds");
+        assert_eq!(url.path(), "/repos/octocat");
+        assert_eq!(
+            url.query(),
+            Some("extra=q-val"),
+            "only the non-placeholder key routes to query"
+        );
+
+        let ctx = noop_context();
+        let schema = json!({
+            "type": "object",
+            "properties": {"id": {"type": "string"}},
+        });
+        let err = build_request(
+            "https://api.example.com",
+            "/items/{id}",
+            "GET",
+            &None,
+            &TestHashMap::new(),
+            "svc",
+            &schema,
+            &json!({"id": "x", "undeclared": "v"}),
+            &ctx,
+        )
+        .expect_err("undeclared key still rejected with a placeholder present");
+        assert_eq!(err.code, "INVALID_INPUT");
+    }
+
+    /// FWD-18, structural-value decision: an object/array value under a
+    /// placeholder key is an `INVALID_INPUT` error, not a spliced JSON
+    /// segment. A path placeholder designates exactly one literal
+    /// segment; the pre-decision behavior embedded the minified JSON
+    /// into the path (and, under an additionalProperties schema, also
+    /// emitted the key as a query param — the double-route).
+    #[test]
+    fn object_or_array_path_values_error_instead_of_splicing_json_into_the_path() {
+        for value in [json!({"a": 1}), json!([1, 2])] {
+            let err = request_url(
+                "https://api.example.com",
+                "/things/{id}",
+                json!({"id": value}),
+            )
+            .expect_err("structural path values must be rejected");
+            assert_eq!(err.code, "INVALID_INPUT", "value was: {value}");
+            assert!(
+                err.message.contains("must be a scalar"),
+                "value was: {value}"
+            );
+            assert!(
+                !err.message.contains("{\"a\":1}") && !err.message.contains("[1,2]"),
+                "error must not echo the structural value: {value}"
+            );
+        }
+
+        for value in [json!("text"), json!(42), json!(true)] {
+            let url = request_url(
+                "https://api.example.com",
+                "/things/{id}",
+                json!({ "id": value }),
+            )
+            .expect("scalar path values still render");
+            assert!(
+                url.path().starts_with("/things/"),
+                "scalar {value} renders into the path"
+            );
+        }
+    }
+
     fn ctx_with_capability(namespace: &str, value: String) -> OperationContext {
         let mut ctx = noop_context();
         ctx.capabilities = Capabilities::new().with_http_token(namespace, value);