[V1] Support long_prefill_token_threshold in v1 scheduler (#15419)

Signed-off-by: Lu Fang <lufang@fb.com>

tests/v1/core/test_scheduler_e2e.py

@@ -0,0 +1,29 @@
+# SPDX-License-Identifier: Apache-2.0
+import os
+
+import pytest
+
+from vllm import LLM
+
+if os.getenv("VLLM_USE_V1", "0") != "1":
+    pytest.skip("Test package requires V1", allow_module_level=True)
+
+MODEL = "meta-llama/Llama-3.2-1B"
+PROMPT = "Hello my name is Robert and I"
+
+
+@pytest.fixture(scope="module")
+def model() -> LLM:
+    return LLM(MODEL,
+               enforce_eager=True,
+               enable_prefix_caching=True,
+               long_prefill_token_threshold=2,
+               max_num_batched_tokens=6,
+               max_num_seqs=3)
+
+
+def test_concurrent_partial_prefill(model):
+    outputs = model.generate([PROMPT] * 3)
+    assert len(outputs) == 3
+    for output in outputs:
+        assert len(output.outputs) == 1