fix: remote-vllm event loop blocking unit test on Mac (#2332)

# What does this PR do?

The remote-vllm `test_chat_completion_doesnt_block_event_loop` unit test
was often failing for me on a Mac with a `httpx.ReadError`. I traced
this back to the swap to the `AsyncOpenAI` client in the remote-vllm
provider as where this started, and it looks like the async client needs
a bit more accurate HTTP request handling from our mock server.

So, this fixes that unit test to send proper Content-Type and
Content-Length headers which makes the `AsyncOpenAI` client happier on
Macs.

## Test Plan

All the test_remote_vllm.py unit tests consistently pass for me on a Mac
now, without any flaking in the event loop one.

`pytest -s -v tests/unit/providers/inference/test_remote_vllm.py`

Signed-off-by: Ben Browning <bbrownin@redhat.com>

tests/unit/providers/inference/test_remote_vllm.py

@@ -69,9 +69,12 @@ class MockInferenceAdapterWithSleep:
             # ruff: noqa: N802
             def do_POST(self):
                 time.sleep(sleep_time)
+                response_body = json.dumps(response).encode("utf-8")
                 self.send_response(code=200)
+                self.send_header("Content-Type", "application/json")
+                self.send_header("Content-Length", len(response_body))
                 self.end_headers()
-                self.wfile.write(json.dumps(response).encode("utf-8"))
+                self.wfile.write(response_body)
 
         self.request_handler = DelayedRequestHandler