fix(test): move final assertion inside async-with to keep 100% cov on 3.11

On Python 3.11 the `async with streamable_http_client(...)` context
manager's `__aexit__` has a coverage-instrumentation quirk (noted in
AGENTS.md) where statements immediately after the `async with` block
are reported as uncovered even when the test itself passes.

Move the `assert isinstance(received, httpx.ConnectError)` inside the
block so coverage hits it deterministically across the 3.10–3.14
matrix. Verified locally against 3.11.

Author: fede-kamel

tests/shared/test_streamable_http.py

@@ -2442,4 +2442,4 @@ async def test_streamable_http_client_real_server_surfaces_network_error(
             request = JSONRPCRequest(jsonrpc="2.0", id=1, method="ping", params={})
             await write_stream.send(SessionMessage(request))
             received = await read_stream.receive()
-    assert isinstance(received, httpx.ConnectError)
+            assert isinstance(received, httpx.ConnectError)