refactor: improve error handling in global context info function

Co-authored-by: Soulter <37870767+Soulter@users.noreply.github.com>

astrbot/core/astr_main_agent.py

@@ -845,14 +845,16 @@ def _apply_global_context_info(event: AstrMessageEvent, req: ProviderRequest) ->
     # Parse the original UMO to extract platform, message type, and session info
     try:
         parts = original_umo.split(":", 2)
-        if len(parts) == 3:
-            platform_id, message_type, session_id = parts
-            context_info = f"[Context: Platform={platform_id}, Type={message_type}, Session={session_id}]"
-            # Prepend context info to the user prompt
-            if req.prompt:
-                req.prompt = f"{context_info} {req.prompt}"
-            else:
-                req.prompt = context_info
+        if len(parts) != 3:
+            logger.warning(
+                f"Original UMO format is invalid (expected 3 parts): {original_umo}"
+            )
+            return
+
+        platform_id, message_type, session_id = parts
+        context_info = f"[Context: Platform={platform_id}, Type={message_type}, Session={session_id}]"
+        # Prepend context info to the user prompt
+        req.prompt = f"{context_info} {req.prompt or ''}"
     except Exception as e:
         logger.warning(f"Failed to parse original UMO for global context: {e}")