feat: upgrade MiniMax default model to M3 (#857)

- Add MiniMax-M3 to the model selection list (set as new default at top)
- Retain MiniMax-M2.7 and MiniMax-M2.7-highspeed
- Remove deprecated MiniMax-M2.5 / M2.5-highspeed
- Update supportsImageInput: M3 supports image input (M2.x stay text-only)
- Update unit tests to reflect new model lineup
- Update example AI_MODEL in CN/EN/JA docs to MiniMax-M3

Co-authored-by: octo-patch <octo-patch@github.com>

lib/ai-providers.ts

@@ -1390,8 +1390,12 @@ export function supportsImageInput(modelId: string): boolean {
         return false
     }
 
-    // MiniMax text models (MiniMax-M2.x series are text-only)
-    if (lowerModelId.includes("minimax") && !hasVisionIndicator) {
+    // MiniMax text models (MiniMax-M2.x series are text-only; M3 supports image input)
+    if (
+        lowerModelId.includes("minimax") &&
+        !hasVisionIndicator &&
+        !lowerModelId.includes("m3")
+    ) {
         return false
     }