Add MiniMax chat provider

[octo-patch]

Summary

• Add and register a MiniMax provider with MiniMax-M3 and MiniMax-M2.7 chat completions.
• Support global and China OpenAI-compatible and Anthropic-compatible endpoints through the provider Base URL.
• Preserve model-specific thinking behavior, context and output limits, and MiniMax-M3 text, image, and video input.
• Add request-capture tests for all four endpoint variants and Anthropic media conversion.

Checks

• python -m unittest llmstack.processors.providers.test_minimax -v
• python3 -m py_compile llmstack/processors/providers/minimax/__init__.py llmstack/processors/providers/minimax/client.py llmstack/processors/providers/minimax/chat_completions.py llmstack/common/utils/sslr/resources/chat/completions.py llmstack/processors/providers/test_minimax.py
• pre-commit run --files llmstack/processors/providers/minimax/__init__.py llmstack/processors/providers/minimax/client.py llmstack/processors/providers/minimax/chat_completions.py llmstack/common/utils/sslr/resources/chat/completions.py llmstack/processors/providers/test_minimax.py

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