feat(ai): add PaLM 2 API support

[genaaredes-ui]

Resolves #279

Summary

• Add a typed Palm2AI client for the PaLM REST API's chat-bison, text-bison, and embedding-gecko endpoints.
• Export the client and its request/response types from the AI package.
• Add a Jsonnet-driven Palm2 example with runtime configuration.
• Add mocked HTTP tests for chat, text generation, embeddings, and input validation.

Tests

• Palm2AI isolated TypeScript check: passed
• Palm2AI Vitest suite: 3 passed
• Palm2 example TypeScript check: passed

The API key is read from the constructor or PALM_API_KEY; no credentials are included in the repository.

[github-actions]

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