test: clarify input-buttons test docstring

gradio silently accepts invalid button values rather than rejecting
them, so the empty-list assertion is the actual guard. Reword the
docstring so it no longer implies a runtime rejection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_app.py

@@ -154,7 +154,7 @@ def test_input_textbox_has_no_placeholder(demo):
 
 
 def test_input_textbox_has_no_buttons(demo):
-    """Input textbox should expose no toolbar buttons (invalid 'clear' button removed)."""
+    """Input textbox should expose no toolbar buttons (gradio accepts invalid button values silently)."""
     textboxes = [b for b in demo.blocks.values() if type(b).__name__ == "Textbox"]
     interactive = [t for t in textboxes if t.interactive is not False]
     assert interactive[0].buttons == [], f"Expected no buttons, got {interactive[0].buttons!r}"