fix(vlm): contain EPD request lifecycle failures (#36944)

Co-authored-by: mickqian <mickqian@users.noreply.github.com>

test/registered/openai_server/basic/test_openai_completion_rust.py

@@ -26,8 +26,7 @@ class TestOpenAICompletionRustParity(CustomTestCase):
     api_key = "sk-123456"
 
     def _get_logprobs(self, *, rust_frontend):
-        # Prefill CUDA graph pads the batch, so numerics follow whichever
-        # requests share the forward pass; the assertions below need equality.
+        # compare identical prefill shapes, without graph padding or warmup cache hits
         process = popen_launch_server(
             self.model,
             DEFAULT_URL_FOR_TEST,
@@ -38,6 +37,7 @@ class TestOpenAICompletionRustParity(CustomTestCase):
                 "--random-seed",
                 "42",
                 "--disable-prefill-cuda-graph",
+                "--disable-radix-cache",
             ],
         )
         try: