feat(AIC-3412): add the Google ADK agents handler

[andrewklatzke]

Summary

• Adds launchdarkly-ai-google-adk-agents, a wildcard agent handler for Google ADK. Gemini Developer API is the default. Vertex is an explicit opt-in and never shares an API key with that path.
• Non-Gemini models go through ADK's own LiteLLM adapter (provider/model). google-adk is not a declared dependency: 2.9.2 caps OpenTelemetry below this workspace's exporter pin, so production installs add google-adk[extensions] themselves.
• Includes google_adk_graph and to_adk_agents. The native adapter runs the root, follows transfer_to_<target> when the model calls it, and emits ld.ai.graph plus $ld:ai:graph:* when a context is present.

Test plan

• uv run mypy packages/google-adk-agents/src
• uv run pytest packages/google-adk-agents/tests tests/test_cross_handler_parity.py
• Live integration against a Google-backed AI Config (the monorepo scenario key is not confirmed Google-backed)
• Confirm a Vertex opt-in call does not send an API key, and a failed Gemini call is not retried on Vertex

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