feat: add google-adk node-weather ai-observability fixture

[marco-g-pm]

Problem

The wizard has no fixture to prove the Google ADK path. PostHog/posthog-js#4687 adds PostHogADKPlugin, and no ai-observability app exercises an ADK Runner plugin.

for PostHog/posthog-js#4687

Changes

• Adds apps/ai-observability/google-adk/node-weather: the same two-turn get_weather round trip as the other weather apps, on @google/adk, so a diff against any of them isolates the framework.
• Its README grades the plugin path: PostHogADKPlugin in the Runner's plugins, identity inherited from ADK's own userId and sessionId, two traces in one session, flush before exit.
• The README also states the plugin emits no tool spans, so a missing $ai_span is not graded as a failure.
• Mechanical: the apps README lists the fixture and adds it to the wrapper-first exceptions.

Testing

• Imports and call shapes match the adk-js source (FunctionTool, Runner, runAsync, createSession) and the working example in feat(ai): add Google ADK (@google/adk) observability adapter posthog-js#4687.
• Not run: tsc --noEmit against installed deps, because registry.npmjs.org is unreachable behind this sandbox's TLS interception.
• Not run: wizard-ci --app ai-observability/google-adk/node-weather, which needs the google-adk variant from feat: add google-adk variant to ai-observability skill context-mill#375.

🤖 Agent context

Written by Claude Code from PostHog/posthog-js#4687, mirroring #3675 (the Go fixture rollout). Skills invoked: writing-pr-descriptions, writing-user-facing-copy, writing-code-comments.