fix(js): finish palm2 and gemini support

[desai2424]

Closes #279

Summary

• add a direct Palm2AI client for Google Generative Language endpoints without the Google SDK
• fix GeminiAI so max_output_tokens, responseType, and related settings are sent in the correct Google request body
• export the Palm2/Gemini types from the AI package
• add server-backed tests under src/ai/src/tests/palm2
• add a Jsonnet-driven example under JS/edgechains/examples/palm2-chat

Root cause

GeminiAI already existed in the tree, but generation settings were attached to the axios config instead of the Google generateContent request body, so they were effectively ignored.

Verification

• ./node_modules/.bin/vitest run src/ai/src/tests/palm2/palm2.test.ts
• npm run build

/claim #279

[github-actions]

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