fix: replace non-inclusive language in optionals.py

Signed-off-by: Luca Muscariello <muscariello@ieee.org>

Author: muscariello

src/a2a/client/optionals.py

@@ -5,12 +5,12 @@
 try:
     from grpc.aio import Channel  # pyright: ignore[reportAssignmentType]
 except ImportError:
-    # If grpc.aio is not available, define a dummy type for type checking.
-    # This dummy type will only be used by type checkers.
+    # If grpc.aio is not available, define a stub type for type checking.
+    # This stub type will only be used by type checkers.
     if TYPE_CHECKING:
 
         class Channel:  # type: ignore[no-redef]
-            """Dummy class for type hinting when grpc.aio is not available."""
+            """Stub class for type hinting when grpc.aio is not available."""
 
     else:
         Channel = None  # At runtime, pd will be None if the import failed.