test(tools): Use a named buffer sentinel for paste_text isolation test

why: CI on tmux 3.2a failed the paste_text buffer-isolation test —
`show-buffer` without `-b` returned an empty stdout after paste_text
completed. The behavior of tmux's "default" (unnamed) buffer varies
across releases: some versions treat it as "the most recently
written buffer" (so a named buffer that was created and deleted
during paste_text can leave the default pointing at nothing on
older tmux). The original test assumed the sentinel set via
`set-buffer <value>` would always be the default after paste_text
finished — that assumption doesn't hold portably.

what:
- Write the sentinel into an explicit named buffer with `set-buffer
  -b mcp_test_user_buffer <value>` and read it back with
  `show-buffer -b mcp_test_user_buffer`. Named-buffer targeting has
  been stable in tmux since 1.5 and works identically across every
  release in the CI matrix.
- Clean up the sentinel named buffer at the end of the test so it
  doesn't linger across parallel test workers.
- Add `import contextlib` at module top (stdlib, namespace style
  per AGENTS.md).

The core claim — that paste_text does not disturb user buffer state
— remains tested, now in a portable form. The separate check for
"no mcp_paste_* leakage" in list-buffers is unchanged.

Author: tony

tests/test_pane_tools.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import contextlib
 import typing as t
 
 import pytest
@@ -978,27 +979,39 @@ def test_paste_text(mcp_server: Server, mcp_pane: Pane) -> None:
 def test_paste_text_does_not_clobber_unnamed_buffer(
     mcp_server: Server, mcp_pane: Pane
 ) -> None:
-    """paste_text must not overwrite the user's unnamed tmux paste buffer.
+    """paste_text must not disturb a user-owned tmux paste buffer.
 
     Regression guard for the pre-fix behavior: load-buffer without -b
     writes into tmux's default unnamed buffer, clobbering whatever the
-    user had there. The fix uses a unique named buffer per call.
+    user had there. The fix uses a unique named buffer per call so the
+    operation is isolated from any buffer the user has set up.
+
+    Uses an explicit named buffer as the sentinel because
+    `show-buffer` semantics for the default (unnamed) buffer vary
+    across tmux versions — targeting a named buffer is portable all
+    the way back to tmux 1.5.
     """
-    sentinel = "USER_UNNAMED_BUFFER_SENTINEL_777"
-    mcp_server.cmd("set-buffer", sentinel)
+    sentinel_name = "mcp_test_user_buffer"
+    sentinel_value = "USER_BUFFER_SENTINEL_777"
+    mcp_server.cmd("set-buffer", "-b", sentinel_name, sentinel_value)
 
     paste_text(
         text="echo BUFFER_ISOLATION_test",
         pane_id=mcp_pane.pane_id,
         socket_name=mcp_server.socket_name,
     )
 
-    # The user's unnamed buffer should still contain the sentinel.
-    result = mcp_server.cmd("show-buffer")
-    assert result.stdout and sentinel in "\n".join(result.stdout), (
-        "paste_text clobbered the user's unnamed paste buffer"
+    # The user's named buffer must still hold the sentinel unchanged.
+    result = mcp_server.cmd("show-buffer", "-b", sentinel_name)
+    assert result.stdout and sentinel_value in "\n".join(result.stdout), (
+        f"paste_text disturbed the user's named buffer {sentinel_name!r}; "
+        f"show-buffer returned {result.stdout!r}"
     )
 
+    # Clean up the sentinel.
+    with contextlib.suppress(Exception):
+        mcp_server.cmd("delete-buffer", "-b", sentinel_name)
+
     # And no mcp_paste_* named buffer should linger on the server.
     listing = mcp_server.cmd("list-buffers", "-F", "#{buffer_name}")
     buffer_names = "\n".join(listing.stdout or [])