fix(minimax): echo reasoning_content on follow-up turns to avoid 400

MiniMax requires reasoning_content echoed back on assistant messages in
multi-turn/tool-call conversations. Add minimax and minimax-cn to
PROVIDER_RULES (scope all), same fix as DeepSeek (#1543).

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/utils/reasoningContentInjector.js

@@ -1,4 +1,4 @@
-// Some thinking-mode providers (DeepSeek, Kimi, ...) require reasoning_content
+// Some thinking-mode providers (DeepSeek, Kimi, MiniMax, ...) require reasoning_content
 // to be echoed back on assistant messages. Clients in OpenAI format don't send it,
 // so we inject a non-empty placeholder to satisfy upstream validation.
 
@@ -6,7 +6,9 @@ const PLACEHOLDER = " ";
 
 // Provider-level rules: keyed by executor.provider
 const PROVIDER_RULES = {
-  deepseek: { scope: "all" }
+  deepseek: { scope: "all" },
+  minimax: { scope: "all" },
+  "minimax-cn": { scope: "all" }
 };
 
 // Model-level rules: matched by predicate against model id