refactor(channels): dedupe transport and gateway test scaffolds

src/browser/server.agent-contract.test-harness.ts

@@ -0,0 +1,26 @@
+import { fetch as realFetch } from "undici";
+import {
+  getBrowserControlServerBaseUrl,
+  installBrowserControlServerHooks,
+  startBrowserControlServerFromConfig,
+} from "./server.control-server.test-harness.js";
+
+export function installAgentContractHooks() {
+  installBrowserControlServerHooks();
+}
+
+export async function startServerAndBase(): Promise<string> {
+  await startBrowserControlServerFromConfig();
+  const base = getBrowserControlServerBaseUrl();
+  await realFetch(`${base}/start`, { method: "POST" }).then((r) => r.json());
+  return base;
+}
+
+export async function postJson<T>(url: string, body?: unknown): Promise<T> {
+  const res = await realFetch(url, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: body === undefined ? undefined : JSON.stringify(body),
+  });
+  return (await res.json()) as T;
+}