feat: add MiniMax provider support to RAG templates

[octo-patch]

Summary

This PR adds MiniMax as an alternative LLM provider option to the Pathway RAG templates.

MiniMax offers an OpenAI-compatible API at https://api.minimax.io/v1, which means it can be used as a drop-in replacement for OpenAI models via pw.xpacks.llm.llms.OpenAIChat by setting base_url and api_key.

Changes

• Added commented MiniMax configuration examples to app.yaml in adaptive_rag, question_answering_rag, multimodal_rag, and slides_ai_search templates
• Added MINIMAX_API_KEY to the corresponding .env.example files

How to use MiniMax

Uncomment the MiniMax section in any template's app.yaml and set MINIMAX_API_KEY in your .env file.

Note: MiniMax requires temperature to be in the range (0.0, 1.0].

API Reference: https://platform.minimax.io/docs/api-reference/text-openai-api

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