[xgtcode]

Summary

Align VeADK Go runner/app usability with VeADK Python while preserving ADK Go’s typed content model and event-stream APIs.

Changes

• Add runner.RunText(...) for plain text input with a final text response.
• Add runner.CollectFinalText(...) to collect final visible model text from a runner event stream.
• Add user content helpers:
  • runner.UserText(...)
  • runner.UserParts(...)
  • runner.UserBytes(...)
  • runner.UserURI(...)
• Add examples/runner/basic to demonstrate:
  • plain text calls with RunText
  • multi-part input with UserParts + CollectFinalText
  • raw event-stream consumption with Run
• Update simple_app to reuse the VeADK runner helper path for /invoke.
• Add docs for:
  • direct runner usage
  • runner.Config vs apps.RunConfig vs agent.RunConfig
  • context.Context vs conversation context
  • Python Runner.run(messages=...) mapping to Go usage
  • app/runner session, memory, and auto-save alignment

Compatibility

This PR is additive and keeps existing runner APIs intact. Run(...) remains the low-level event-stream API. Go continues to use typed genai.Content / genai.Part values instead of introducing a Python-style messages any API.

Testing

go test ./apps/simple_app ./apps ./apps/agentkit_server_app ./runner ./examples/runner/basic