Add Google PaLM2 API support

[alceops]

Closes #279.

Summary

• Adds a typed Palm2AI client for Google PaLM2 text-bison and chat-bison endpoints.
• Exports PaLM2 classes/types from the AI package entrypoint.
• Adds unit testcases in a testcases/palm2 folder covering text and chat request payloads.
• Adds a PaLM2 example with prompts/config kept in examples/palm2/prompt.jsonnet instead of hardcoded prompt strings.

Verification

• npm run build
• npx vitest run src/ai/src/testcases/palm2/palm2.test.ts

[alceops]

Quick verification pass for maintainer review: this PR is still scoped to PaLM2 API support only, keeps prompt content in the Jsonnet example, and was verified locally with npm run build plus npx vitest run src/ai/src/testcases/palm2/palm2.test.ts. I can turn around any requested changes quickly.