[MPS] Add StreamContext stub (#20782)

Signed-off-by: Xiaodong Ye <yeahdongcn@gmail.com>

python/sglang/_mps_stub.py

@@ -69,6 +69,19 @@ class Event:
         return 0.0
 
 
+class StreamContext:
+    """Minimal stand-in for ``torch.cuda.StreamContext``."""
+
+    def __init__(self, stream: Any = None) -> None:
+        pass
+
+    def __enter__(self) -> "StreamContext":
+        return self
+
+    def __exit__(self, *args: Any) -> None:
+        pass
+
+
 _default_stream = Stream()
 
 
@@ -235,6 +248,7 @@ def install() -> None:
     # Only patch attributes that are actually missing
     for name, obj in [
         ("Stream", Stream),
+        ("StreamContext", StreamContext),
         ("Event", Event),
         ("current_stream", current_stream),
         ("stream", stream),