Check FastAPI package on A2AFastAPIApplication()

On app initialization with app = A2aFastAPIApplication(), check that the
FastAPI package is installed and raise ImportError with the custom
messsage if necessary.

Author: darkhaniop

src/a2a/server/apps/jsonrpc/fastapi_app.py

@@ -52,6 +52,17 @@ def __init__(
             context_builder=context_builder,
         )
 
+        self._check_fastapi_dependency()
+
+    def _check_fastapi_dependency(self) -> None:
+        """Checks if the FastAPI package is installed.
+
+        If instead of the actual FastAPI class, a dummy implementation from
+        ./fastapi_import_helpers.py is imported, initializing FastAPI() would
+        raise ImportError.
+        """
+        _app = FastAPI()
+
     def add_routes_to_app(
         self,
         app: FastAPI,