Seal the fetch target: one value holds the URL and its host

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/brokerd/tests/args.rs

@@ -289,16 +289,16 @@ fn each_tool_parses_its_own_arguments() {
             cwd: Some("/home/kyle".to_string())
         })
     );
-    assert_eq!(
-        parse(
-            ToolName::HttpFetch,
-            r#"{"url":"https://www.example.com/a"}"#
-        ),
-        Ok(ToolArgs::HttpFetch {
-            url: "https://www.example.com/a".to_string(),
-            host: "www.example.com".to_string()
-        })
+    // A `FetchUrl` cannot be built outside `args`, so the parsed value is read through its getters.
+    let fetched = parse(
+        ToolName::HttpFetch,
+        r#"{"url":"https://www.example.com/a"}"#,
     );
+    let Ok(ToolArgs::HttpFetch(target)) = fetched else {
+        panic!("{fetched:?}")
+    };
+    assert_eq!(target.url(), "https://www.example.com/a");
+    assert_eq!(target.host(), "www.example.com");
     // Field order and white space in the request do not matter.
     assert_eq!(
         parse(