test(core,csr): replace text-substring agreement check with a behavioural one

The old assertions only searched REACTIVE_RUNTIME for substrings; they never
touched buildApiRequest and were not anchored to the content-type line they
claimed to guard, so they could not detect drift on either side. Replace
with a fixture-driven test that runs both implementations on the same
(path, method, input) cases and compares the actual request they produce.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/core/test/api-request.test.ts

@@ -1,6 +1,5 @@
 import { expect, test } from "bun:test";
 import { buildApiRequest } from "../src/api-request.ts";
-import { REACTIVE_RUNTIME } from "../../csr/src/reactive-runtime.ts";
 
 test("GET builds a query string", () => {
   expect(buildApiRequest("/api/users", "GET", { name: "Ajay" }).url).toBe("/api/users?name=Ajay");
@@ -47,9 +46,7 @@ test("values are encoded", () => {
   );
 });
 
-test("the browser runtime and the shared builder agree", () => {
-  // Cheap structural guard: the runtime must apply the same omission rule.
-  // If someone changes one side's rules, this fails.
-  expect(REACTIVE_RUNTIME).toContain('value === ""');
-  expect(REACTIVE_RUNTIME).toContain("application/json");
-});
+// The behavioural agreement test between this builder and the browser
+// runtime's wrnexusCallApi lives in packages/csr/test/api-request-agreement.test.ts,
+// which can host the happy-dom harness needed to run the runtime and compare
+// outputs. This file has no DOM available.