fix: make test context http.fetch use globalThis.fetch for proper mocking

The test context's http.fetch was hardcoded to return an empty Response(),
preventing proper mocking of globalThis.fetch in tests. Now it delegates
to globalThis.fetch so test mocks can properly intercept HTTP calls.

Signed-off-by: betterclever <[email protected]>
Amp-Thread-ID: https://ampcode.com/threads/T-019c1a9e-e1f3-755b-99e7-5a617c149f9e
Co-authored-by: Amp <[email protected]>

Author: betterclever

worker/src/components/ai/__tests__/ai-agent.test.ts

@@ -53,7 +53,7 @@ function createTestContext(overrides?: Partial<ExecutionContext>): ExecutionCont
       },
     },
     http: {
-      fetch: async () => new Response(),
+      fetch: (url, init) => globalThis.fetch(url, init),
       toCurl: () => '',
     },
     ...overrides,