Add PaLM2-compatible Google AI support

[Luvi-1]

/claim #279

Summary

• export a typed Palm2AI compatibility class from the JavaScript AI package
• repair Google generation settings so they are sent in the generationConfig request body
• allow model and API-base selection while defaulting to Google's maintained Generative Language endpoint
• add mocked coverage under tests/palm2 with its prompt loaded from testcases/palm2/prompt.jsonnet
• add a Jsonnet-driven palm2-chat example with no prompt hardcoded in TypeScript

The original PaLM models are retired; this preserves the requested class name on Google's maintained generateContent API instead of shipping an example against a dead model endpoint.

Validation

• npx tsc -p tsconfig.json --noEmit --incremental false
• npx vitest run src/ai/src/tests/palm2/palm2.test.ts (1 passed)
• example TypeScript check passed
• repository Prettier settings passed for every touched file

Closes #279

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