Preserve exception chains in AsgiFastStream startup

faststream/asgi/app.py

@@ -246,7 +246,7 @@ async def start_lifespan_context(
 
             except ExceptionGroup as e:
                 for ex in e.exceptions:
-                    raise ex from None
+                    raise ex from ex.__cause__
 
     async def __start(
         self,

tests/asgi/test_lifespan.py

@@ -0,0 +1,25 @@
+import pytest
+from starlette.testclient import TestClient
+
+from faststream.asgi import AsgiFastStream
+
+
+@pytest.mark.asyncio()
+async def test_chained_exception_cause_preserved_on_startup_failure() -> None:
+    """AsgiFastStream must not discard __cause__ when re-raising startup exceptions unwrapped from anyio's ExceptionGroup.
+
+    This is a regression test for https://github.com/ag2ai/faststream/issues/2780
+    """
+    app = AsgiFastStream()
+
+    cause = ConnectionError("underlying cause")
+
+    @app.on_startup
+    async def failing_startup() -> None:
+        msg = "application startup failed"
+        raise RuntimeError(msg) from cause
+
+    with pytest.raises(RuntimeError) as exc_info, TestClient(app):
+        pass
+
+    assert exc_info.value.__cause__ is cause