test: drop an unnecessary double cast in the relay progress test helper (#2028)

The backendHoldingSend fetch mock is already declared with vi.fn<typeof fetch>(),
so the trailing `as unknown as typeof fetch` was redundant and tripped the
repo's no-double-cast rule. Return the typed mock directly; the test project
typechecks without it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PsmUAZ6aLUAFqenjJKSSRp
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/test/core/mcp/remote/remoteClientTransport.test.ts

@@ -723,7 +723,7 @@ describe("RemoteClientTransport", () => {
             return new Response(JSON.stringify({ ok: true }), { status: 200 });
           }
           return new Response("not found", { status: 404 });
-        }) as unknown as typeof fetch;
+        });
       return { fetchFn, getSse: () => sse, getSentId: () => sentId };
     }