feat: Sync from internal staging.

[chengfeitao-google]

PiperOrigin-RevId: 968071773

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feat(samples): update sample apps and backend for template agent routing and offline testing

Features & Capabilities

• Template Agent Support & Routing:

  • Backend Server (agent/python/):
    • Integrated template-based agent execution into the Python backend server.
    • Added prefix-based routing ([TEMPLATE]) to automatically direct user queries to the template agent pipeline.
    • Enabled configurable agent selection and fallback modes via CLI flags and environment variables.
    • Configured default fallback mode to grounded text response to significantly reduce latency and eliminate hallucinations when template extraction does not apply.
  • Mobile Clients (iOS & Android):
    • Added a "Template Agent" toggle in both the iOS and Android demo applications.
    • Automatically prefixes outbound requests with [TEMPLATE] when enabled to route traffic on the backend.

• Offline & Mock Testing (iOS):

  • Added a "Use Canned Responses" switch in the iOS sample app to run completely offline without an active backend server.
  • Introduced MockScenarioRegistry to centrally map UI demo buttons and automated UI test cases to local fixture files.
  • Updated UI test suites to use the scenario registry for consistent offline verification.

CI / CD & Maintenance

• Dependabot: Updated .github/dependabot.yml configuration and formatting to ensure dependency automation operates smoothly.