Delete the generated spec assets in the agent SDK

google#760 accidentally checked in obsolete
and generated assets.
This commit removes them and also updates the git ignore file to ignore
them when checking in code.

Author: nan-yu

.gitignore

@@ -15,3 +15,7 @@ site/
 
 # Python virtual environment
 .venv/
+
+# Generated spec assets in the agent SDK
+a2a_agents/python/a2ui_agent/src/a2ui/assets/**/*.json
+agent_sdks/python/src/a2ui/assets/**/*.json