feat: upgrade MiniMax default model to M2.7 (#13676)

## Summary
Upgrade MiniMax model configuration to include the latest M2.7 model.

## Changes
- Add `MiniMax-M2.7` and `MiniMax-M2.7-highspeed` to the model selection
list in `conf/llm_factories.json`
- Place M2.7 models at the top of the list as the recommended default
- Retain all previous models (M2.5, M2.5-highspeed, M2.1, M2) as
available alternatives

## Why
MiniMax-M2.7 is the latest flagship model with enhanced reasoning and
coding capabilities. This update ensures RAGFlow users can access the
newest model while maintaining backward compatibility with existing
configurations.

## Testing
- JSON config validated (well-formed)
- No existing MiniMax-specific unit tests affected
- Model entries follow the same structure as existing entries

Co-authored-by: PR Bot <pr-bot@minimaxi.com>

conf/llm_factories.json

@@ -1300,6 +1300,20 @@
             "rank": "810",
             "url": "https://api.minimaxi.com/v1",
             "llm": [
+                {
+                    "llm_name": "MiniMax-M2.7",
+                    "tags": "LLM,CHAT,200k",
+                    "max_tokens": 204800,
+                    "model_type": "chat",
+                    "is_tools": true
+                },
+                {
+                    "llm_name": "MiniMax-M2.7-highspeed",
+                    "tags": "LLM,CHAT,200k",
+                    "max_tokens": 204800,
+                    "model_type": "chat",
+                    "is_tools": true
+                },
                 {
                     "llm_name": "MiniMax-M2.5",
                     "tags": "LLM,CHAT,200k",