ControlMode(capture): Trim trailing whitespace-only lines from capture_pane output

why: Control mode returns viewport-sized blank lines; align capture-pane output with subprocess expectations.
what:
- Drop trailing whitespace-only lines in Pane.capture_pane output
- Lint/format/type/tests remain green

Author: tony

src/libtmux/pane.py

@@ -349,7 +349,11 @@ def capture_pane(
             cmd.extend(["-S", str(start)])
         if end is not None:
             cmd.extend(["-E", str(end)])
-        return self.cmd(*cmd).stdout
+        output = self.cmd(*cmd).stdout
+        # Control mode (and tmux in general) can include viewport-sized blank lines.
+        while output and output[-1].strip() == "":
+            output.pop()
+        return output
 
     def send_keys(
         self,