Add Google ADK (Gemini) agent samples

[maxjneto]

What was changed

Added google_adk/ — a new sample directory demonstrating the newest temporalio[google-adk] integration with Google's Agent Development Kit (Gemini models).

Three sub-examples:

• basic/ — Hello world agent + activity-backed tools via activity_tool
• orchestration/ — Sequential pipeline, parallel fan-out, and iterative loop patterns
• human_in_the_loop/ — Signal-based approval before sensitive tool execution

Also updated:

• pyproject.toml — added google-adk dependency group
• README.md — added google_adk to the samples list

Why?

The openai_agents/ sample demonstrates the OpenAI Agents SDK integration, but there's no equivalent for the Google ADK integration (temporalio.contrib.google_adk_agents). This fills that gap, showing users how to use TemporalModel, activity_tool, GoogleAdkPlugin, and workflow primitives (signals, queries) with ADK agents.

Checklist

1. Closes no existing issue, happy to open one if preferred

2. How was this tested:

   • uv run ruff check google_adk/ — passes
   • uv run ruff format --check google_adk/ — passes
   • uv run mypy --check-untyped-defs --namespace-packages google_adk/ — passes (0 errors, 19 files)
   • Manual run with local Temporal server + GOOGLE_API_KEY set:

     uv sync --group google-adk
     # Terminal 1
     uv run google_adk/basic/run_worker.py
     # Terminal 2
     uv run google_adk/basic/run_hello_world_workflow.py
     

3. Any docs updates needed?

   • README in google_adk/README.md is included with full instructions
   • Root README.md updated with one-line listing
   • No docs.temporal.io changes needed unless the team wants to link to it from the google-adk contrib module docs

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