Enable Rich UI embed support for action functions (#21294)

Action functions can now return HTMLResponse objects or (html, headers)
tuples with Content-Disposition: inline to render rich UI iframes in
chat, matching the existing tool behavior.

https://claude.ai/code/session_01KCZKQXj1uqgPjqjMd4U2NF

Co-authored-by: Claude <noreply@anthropic.com>

backend/open_webui/utils/middleware.py

@@ -859,7 +859,7 @@ def process_tool_result(
         else:
             tool_result = tool_result.body.decode("utf-8", "replace")
 
-    elif (tool_type == "external" and isinstance(tool_result, tuple)) or (
+    elif (tool_type in ("external", "action") and isinstance(tool_result, tuple)) or (
         direct_tool and isinstance(tool_result, list) and len(tool_result) == 2
     ):
         tool_result, tool_response_headers = tool_result