fix(attention): read per-runner kv cache dtype off model_runner (#32251)

test/manual/attention/test_flashattn_backend.py

@@ -50,13 +50,13 @@ class MockModelRunner:
         self.kv_cache_dtype = (
             self.dtype
         )  # torch dtype, required by FlashAttentionBackend
+        self.kv_cache_dtype_str = "auto"
 
-        # server_args is still needed for string-based config (kv_cache_dtype_str)
         self.server_args = type(
             "ServerArgs",
             (),
             {
-                "kv_cache_dtype": "auto",  # string version for kv_cache_dtype_str
+                "kv_cache_dtype": "auto",
                 "speculative_eagle_topk": None,
                 "speculative_num_draft_tokens": 0,
                 "enable_deterministic_inference": False,

test/manual/attention/test_flashattn_mla_backend.py

@@ -49,6 +49,7 @@ class MockModelRunner:
             },
         )
         self.kv_cache_dtype = self.server_args.kv_cache_dtype
+        self.kv_cache_dtype_str = "auto"
 
         batch_size = 160
         # Create a proper req_to_token_pool with the req_to_token attribute