feat: add MiniMax as alternative LLM provider for RAG dataflows

[octo-patch]

Summary

Add MiniMax as an alternative LLM provider alongside OpenAI in both faiss_rag and conversational_rag contrib dataflows, using Hamilton's native @config.when pattern for provider switching.

• Add MiniMax M2.7 (1M token context window) via OpenAI-compatible API
• Use @config.when_not(provider="minimax") for OpenAI default (backward-compatible)
• Use @config.when(provider="minimax") for MiniMax provider variant
• Update valid_configs.jsonl, tags.json, and README.md for both dataflows
• Add 35 unit tests + 6 integration tests (all passing)

Usage

Switch to MiniMax by setting MINIMAX_API_KEY and passing {"provider": "minimax"} in config:

from hamilton import driver
dr = (
    driver.Builder()
    .with_modules(faiss_rag)
    .with_config({"provider": "minimax"})  # or {} for default OpenAI
    .build()
)

Why MiniMax?

MiniMax offers high-performance models with large context windows (up to 1M tokens) via an OpenAI-compatible API, making it a drop-in alternative for OpenAI in RAG pipelines. The M2.7 model provides strong reasoning capabilities at competitive pricing.

Files Changed (10 files)

faiss_rag (5 files):

• init.py: Multi-provider LLM client + response via @config.when
• README.md: MiniMax usage docs and config table
• valid_configs.jsonl: Added minimax config
• tags.json: Added minimax tag
• test_faiss_rag.py: 17 unit + 3 integration tests

conversational_rag (5 files):

• init.py: Multi-provider LLM client, standalone_question, response via @config.when
• README.md: MiniMax usage docs and config table
• valid_configs.jsonl: Added minimax config
• tags.json: Added minimax tag
• test_conversational_rag.py: 18 unit + 3 integration tests

Test plan

• All 35 unit tests pass (mocked LLM clients)
• All 6 integration tests pass (real MiniMax API calls)
• Default config ({}) still resolves to OpenAI (backward compatible)
• {"provider": "minimax"} correctly resolves to MiniMax M2.7
• Hamilton driver builds successfully with both configs
• Verify CI passes on PR