add an `examples` app that has a gallery of pre-designed surfaces and allows editing/previewing A2UI JSON #4040

Job	Run time
matrix	8s
packages_genui_a2a (beta)	1m 39s
examples_composer (stable)	1m 34s
packages_genui_a2a (stable)	1m 42s
examples_travel_app (beta)	1m 53s
packages_genui (beta)	2m 23s
packages_genai_primitives (stable)	1m 20s
examples_simple_chat (beta)	1m 42s
packages_genui (stable)	2m 0s
examples_composer (beta)	1m 39s
examples_verdure_client (beta)	1m 35s
examples_verdure_client (stable)	1m 40s
examples_catalog_gallery (beta)	1m 52s
tool_release (stable)	1m 13s
examples_travel_app (stable)	2m 2s
packages_json_schema_builder (stable)	1m 15s
packages_json_schema_builder (beta)	1m 26s
tool_test_and_fix (beta)	1m 12s
tool_test_and_fix (stable)	1m 14s
examples_simple_chat (stable)	1m 54s
tool_fix_copyright (stable)	1m 19s
tool_release (beta)	1m 14s
tool_fix_copyright (beta)	1m 22s
examples_catalog_gallery (stable)	1m 37s
packages_genai_primitives (beta)	1m 2s
	37m 57s

Provide feedback