feat: add OpenChat prompt template

[MukundaKatta]

Fixes #245.

Adds an OpenChat prompt template provider for the documented GPT4 Correct conversation format:

• text, instruction, and chat prompt mappings
• system prompt prefix support
• end-of-turn stop sequence
• chat prompt validation for alternating user/assistant turns
• snapshots covering each prompt shape

Validation:

• pnpm --filter modelfusion exec vitest --config vitest.config.js --run src/model-function/generate-text/prompt-template/OpenChatPromptTemplate.test.ts
• pnpm exec prettier --check packages/modelfusion/src/model-function/generate-text/prompt-template/OpenChatPromptTemplate.ts packages/modelfusion/src/model-function/generate-text/prompt-template/OpenChatPromptTemplate.test.ts packages/modelfusion/src/model-function/generate-text/prompt-template/index.ts