Pane(fix[display_popup]): raise ValueError when close_on_exit + close_on_success combined

why: tmux counts -E flags via args_count(); 3x -E evaluates as != 2, falls
     through to the args_has() branch and silently behaves like 1x -E
     (close-on-any-exit), discarding the close_on_success intent entirely.
what:
- Add mutual-exclusion guard: raise ValueError when both close_on_exit and
  close_on_success are True
- Assign message to variable first to satisfy EM101/TRY003 linting rules

Author: tony

src/libtmux/pane.py

@@ -1256,6 +1256,14 @@ def display_popup(
         if close_existing:
             tmux_args += ("-C",)
 
+        if close_on_exit and close_on_success:
+            msg = (
+                "close_on_exit and close_on_success are mutually exclusive: "
+                "use close_on_exit=True for -E (close on any exit) "
+                "or close_on_success=True for -EE (close on zero exit code only)"
+            )
+            raise ValueError(msg)
+
         if close_on_exit:
             tmux_args += ("-E",)