Add MiniMax as an OpenAI-compatible model provider (#1120)

* Add MiniMax as an OpenAI-compatible model provider

MiniMax offers high-performance LLMs (M2.5, M2.5-highspeed) with
204K context windows. This commit adds MiniMax as a selectable
provider in the configuration system.

Changes:
- Add MiniMax to SUPPORTED_MODELS with model definitions
- Add MiniMax provider configuration in conf/config.yaml
- Update documentation with MiniMax setup instructions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Update README to remove MiniMax API details

Removed mention of MiniMax API usage and configuration examples.

---------

Co-authored-by: octo-patch <octo-patch@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

config.example.yaml

@@ -106,6 +106,29 @@ models:
   #       thinking:
   #         type: enabled
 
+  # Example: MiniMax (OpenAI-compatible)
+  # MiniMax provides high-performance models with 204K context window
+  # Docs: https://platform.minimax.io/docs/api-reference/text-openai-api
+  # - name: minimax-m2.5
+  #   display_name: MiniMax M2.5
+  #   use: langchain_openai:ChatOpenAI
+  #   model: MiniMax-M2.5
+  #   api_key: $MINIMAX_API_KEY
+  #   base_url: https://api.minimax.io/v1
+  #   max_tokens: 4096
+  #   temperature: 1.0  # MiniMax requires temperature in (0.0, 1.0]
+  #   supports_vision: true
+
+  # - name: minimax-m2.5-highspeed
+  #   display_name: MiniMax M2.5 Highspeed
+  #   use: langchain_openai:ChatOpenAI
+  #   model: MiniMax-M2.5-highspeed
+  #   api_key: $MINIMAX_API_KEY
+  #   base_url: https://api.minimax.io/v1
+  #   max_tokens: 4096
+  #   temperature: 1.0  # MiniMax requires temperature in (0.0, 1.0]
+  #   supports_vision: true
+  
   # Example: OpenRouter (OpenAI-compatible)
   # OpenRouter models use the same ChatOpenAI + base_url pattern as other OpenAI-compatible gateways.
   # - name: openrouter-gemini-2.5-flash