Add a google_genai_plugin/ sample suite for temporalio.contrib.google_genai,
mirroring the strands_plugin/ layout (one feature per sub-directory, each with
workflow.py / run_worker.py / run_workflow.py / README.md).

Samples cover every major plugin feature:
- hello_world: generate_content
- tools: automatic function calling (activity_as_tool + plain workflow-method tool)
- streaming: generate_content_stream + streaming_topic/WorkflowStream
- chat: multi-turn client.chats
- structured_output: response_schema + Pydantic
- mcp: TemporalMcpClientSession with a local echo MCP server
- files: client.files.upload (live API)
- interactions: client.interactions stateful API (live API)
- agents: client.agents CRUD (live API)
- vertex_ai: vertexai=True configuration (GCP credentials)

Tests under tests/google_genai_plugin/ use the plugin's GeminiTestServer to run
the model-layer samples offline; the mcp test additionally registers a real
echo MCP server. files/interactions/agents/vertex_ai are runnable-only (require
live credentials) and documented as such.

Register the suite in pyproject.toml (google-genai dependency group + wheel
package), the root README, and CODEOWNERS.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Wrap the workflow.py, run_worker.py, and run_workflow.py bodies of each
sample in @@@SNIPSTART/@@@SNIPEND markers (python-google-genai-<sample>-<part>)
so the code can be embedded in docs, matching the strands_plugin convention.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>