fix(google-adk): handle OpenUI actions correctly

[vishxrad]

What changed

• normalize OpenUI action messages before forwarding them to Google ADK
• preserve submitted form values and send ADK events with the user role
• support weather lookups for one or multiple cities with clearer agent instructions
• surface model, quota, and authentication failures as visible OpenUI callouts
• update the Google ADK example documentation to match the implementation

Why

OpenUI-generated actions could lose the user's intended request before reaching the ADK agent. This caused the model to repeatedly generate onboarding or weather-search forms instead of invoking the weather tool and answering the request.

Impact

Prompts and UI actions now reach the ADK agent consistently, form submissions retain their values, multi-city requests are supported, and backend failures are visible in the rendered conversation.

Validation

• TypeScript checks
• lint and formatting checks
• git diff --check
• local end-to-end weather request verified in the example UI