feat: add Google PaLM 2 API support

[cwjcwj95]

Summary

This PR adds Google PaLM 2 API support to the EdgeChains JavaScript SDK.

Changes

• Palm2AI class (lib/palm2/palm2.ts): Core API client with generateText, chat, and generateEmbeddings methods
• TypeScript types (types/index.ts): Added Palm2Model union type for all supported models
• Unit tests (tests/palm2.test.ts): 8 test cases covering text generation, default params, error handling, retry logic, embeddings, and chat alias
• Jsonnet example (examples/palm2-chat/): Complete example with main.jsonnet, secrets.jsonnet, TypeScript callback, and README
• Export (index.ts): Exported Palm2AI from main entry point

Supported Models

• text-bison-001 (default)
• text-bison-002
• text-bison
• text-unicorn-001
• chat-bison-001
• chat-bison-002
• chat-bison

API Usage

import { Palm2AI } from "@arakoodev/edgechains.js/ai";

const palm2 = new Palm2AI({ apiKey: "your-api-key" });
const response = await palm2.chat({ prompt: "Hello, how are you?" });
console.log(response.content);

Closes #279

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