pane(fix): Unify capture_pane trimming with engine behavior

Change _trim() in capture_pane() from `line.strip() == ""` to `line == ""`
to match the trimming behavior in subprocess_engine.py and control_protocol.py.

The previous `.strip()` approach was too aggressive, removing lines that
contain only whitespace (like a shell prompt `$`), causing mismatches
between subprocess and control mode output.

Author: tony

src/libtmux/pane.py

@@ -353,8 +353,9 @@ def capture_pane(
         output = self.cmd(*cmd).stdout
 
         def _trim(lines: list[str]) -> list[str]:
+            # Match engine trimming: remove only empty strings, not whitespace-only
             trimmed = list(lines)
-            while trimmed and trimmed[-1].strip() == "":
+            while trimmed and trimmed[-1] == "":
                 trimmed.pop()
             return trimmed