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>

backend/docs/CONFIGURATION.md

@@ -41,6 +41,25 @@ models:
         thinking:
           type: enabled
 
+  - 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
   - name: openrouter-gemini-2.5-flash
     display_name: Gemini 2.5 Flash (OpenRouter)
     use: langchain_openai:ChatOpenAI