feat(node): add vercel ai otel providers

[richardsolomou]

Context

Adds Vercel AI SDK OTEL provider variants to the Node test app so we can validate PostHog AI telemetry behavior (including multi-part generation tracing and embeddings/image flows) through concrete provider selections.

What Changed

• Added new provider implementations:
  • VercelAIOtelOpenAIProvider
  • VercelAIOtelOpenAIEmbeddingsProvider
  • VercelAIOtelGeminiImageProvider
• Wired new providers into CLI provider selection and test-mode filtering:
  • Added menu options 24, 25, 26
  • Included 25 in embeddings mode options
  • Included 26 in image mode options
  • Updated provider selection validation and prompt range to 1-26
• Added required OTEL deps in node/package.json:
  • @opentelemetry/api
  • @opentelemetry/sdk-node

Why

This gives us dedicated OTEL-enabled provider paths to verify how experimental_telemetry spans are exported and mapped into PostHog AI events for:

• chat/tool-call flows
• embedding calls
• Gemini image generation

Validation

• Build passes:
  • pnpm --dir node build
• Manual checks performed while testing:
  • providers are selectable in CLI
  • OTEL provider flows emit telemetry events with expected function IDs/metadata

Notes

• VercelAIOtelOpenAIProvider currently uses a forced two-part response path for tracing validation (part1 + part2).
• VercelAIOtelGeminiImageProvider is configured for single generateText image generation call.