fix(ControlMode): handle -P flag output with % prefix correctly

Root Cause:
- tmux pane IDs use format %N (like %0, %1, %2, %3)
- When split-window -P -F#{pane_id} outputs "%3", it's sent INSIDE
  the %begin/%end command block
- But because it starts with %, the protocol parser treated it as
  a control message/notification, NOT as stdout content
- Result: pane_cmd.stdout was empty, causing IndexError

The Fix:
- Modified _handle_percent_line() to check parser state
- When IN_COMMAND state, lines starting with % that aren't known
  control messages (%begin, %end, %error) are now treated as stdout
- This correctly captures pane/window/session IDs from -P flag

Impact:
- Fixed 62 of 82 test failures (76% reduction)
- Before: 82 failed, 512 passed
- After: 20 failed, 574 passed
- All split/resize/pane operations now work correctly

Remaining Issues:
- capture-pane -p still has issues (different root cause)
- Environment variable passing needs investigation
- Session name validation edge cases
- Some doctests need updates

Verification:
Confirmed with debug logging that %3␞ was being sent inside command
block but incorrectly routed to notification handler instead of stdout.

Related: #605

Author: tony

src/libtmux/_internal/engines/control_protocol.py

@@ -188,6 +188,12 @@ def _handle_percent_line(self, line: str) -> None:
             self._on_begin(parts)
         elif tag in ("%end", "%error"):
             self._on_end_or_error(tag, parts)
+        elif self.state is ParserState.IN_COMMAND and self._current:
+            # Inside a command block, lines starting with % that aren't
+            # control messages (begin/end/error) are likely tmux identifiers
+            # (pane IDs like %3, window IDs like @2, etc.) from -P flag output
+            # Treat them as stdout content, not notifications
+            self._current.stdout.append(line)
         else:
             self._on_notification(line, parts)