chore: Add note for multi-providers and installing provider packages

[jsonbailey]

Note

Adds README sections detailing how to install provider-specific packages (e.g., OpenAI) for LangChain and Vercel AI SDK integrations with links to provider docs.

• Docs:
  • packages/ai-providers/server-ai-langchain/README.md:
    • Add “Langchain Provider Installation” section explaining need for installing provider-specific packages and example for @langchain/openai, plus link to integrations docs.
  • packages/ai-providers/server-ai-vercel/README.md:
    • Add “Vercel AI Provider Installation” section explaining provider package requirements and example for @ai-sdk/openai, plus link to providers docs.

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