contrib/google_adk_agents: rename activity_tool to activity_as_tool - #1735

BREAKING CHANGE: `temporalio.contrib.google_adk_agents.workflow.activity_tool`
is renamed to `activity_as_tool`, with no compatibility alias.

Every other Temporal plugin that wraps an activity as an agent tool names
the helper `activity_as_tool`: openai_agents, strands (plus
`activity_as_hook`), and google_genai. google_adk_agents was the only
outlier at `activity_tool`.

The divergence is also cross-language. sdk-go's plugin for the same
framework already uses `ActivityAsTool`, with `ActivityToolOptions` for
the options struct and `activityTool` for the private impl type -- the
verb names the conversion, the noun names the result. Python used the
noun for the verb. sdk-typescript uses `activityAsTool` in its
openai-agents and strands plugins. Left alone, a Go user and a Python
user reading Temporal's Google ADK guide see different names for the
same primitive.

No compatibility alias. The ADK integration is Pre-release at every
level -- the docs page carries a prerelease banner, the 1.24.0 release
notes list it under Pre-release, and the function docstring warns it is
experimental -- and Temporal's published policy for that stage is
"Experimental; API is subject to change". This matches how prior plugin
renames landed (#1139 made `set_open_ai_agent_temporal_overrides`
private, #947 privatized the openai_agents module layout), neither of
which shipped an alias.

Mechanical only. `**kwargs: Any` on this helper is still out of step with
the siblings, which enumerate activity options explicitly or take an
`ActivityConfig`, and the docstring's "Decorator/Wrapper" claim is wrong
(`@activity_as_tool(...)` parameterized use fails). Both are left for a
follow-up so this diff stays reviewable as a rename.

Docs and samples PRs follow once this merges: temporalio/documentation
(docs/develop/python/integrations/google-adk.mdx), samples-python
(google_adk_agents/), and google/adk-docs
(docs/integrations/temporal.md).