[AI-163] google_adk_agents: support ToolContext session state in activity_tool

[DABH]

What was changed

Adds ToolContextSnapshot to temporalio.contrib.google_adk_agents.workflow: a frozen, serializable dataclass carrying the session state (as a plain dict) and the function-call id of an ADK ToolContext.

An activity wrapped with activity_tool can now declare a parameter named tool_context annotated with ToolContextSnapshot:

@activity.defn
async def get_weather(query: str, tool_context: ToolContextSnapshot) -> dict:
    db_url = tool_context.state.get("url", "")
    ...

weather_tool = activity_tool(get_weather, start_to_close_timeout=timedelta(seconds=30))

Exactly like a native ADK function tool's tool_context parameter, it is excluded from the LLM-facing tool schema (ADK reserves the parameter name) and filled in at invocation time — the wrapper snapshots the live ToolContext workflow-side before scheduling the activity. Annotating the parameter with a live ADK context type raises an actionable ValueError at wrap time instead of a serialization failure at runtime.

Why

Closes #1470 (JIRA AI-163). The reporter's tools read configuration seeded into session state (tool_context.state.get("key")), which previously required hand-writing a plain tool function around each activity.

The design follows the constraints laid out in the issue discussion:

• The live ToolContext never crosses the activity boundary — activity inputs go to the server and may run on a different worker, so only the explicitly-called-out serializable subset is passed, extracted workflow-side before invocation.
• Nothing leaks into the tool schema — the parameter rides ADK's own tool_context name-reservation, so declarations exclude it on both the legacy and JSON-schema paths.
• Scoped to ToolContext.state (plus the function-call id), as a single input object so more fields can be added later without breaking activity signatures.
• One-way by design — mutations inside the activity don't propagate; session-state modifications must happen workflow-side from information the activity returns. Documented in the class docstring and README.

Checklist

1. Closes [Feature Request] Support ToolContext for ADK function tools #1470
2. How was this tested: new tests/contrib/google_adk_agents/test_adk_tool_context.py — end-to-end workflow run with max_cached_workflows=0 (state read from a seeded session, default parameter handling, populated function-call id, tool_context mid-signature to prove positional slotting), local-ADK-run parity outside a workflow, schema-exclusion assertions for tools with and without the parameter, and wrap-time rejection of ADK-context / unrelated annotations. Full contrib suite: 23 passed, 5 skipped (pre-existing). pyright/basedpyright/mypy/ruff/pydocstyle clean.
3. Any docs updates needed: contrib README updated in this PR.