ControlMode(test[regressions]): Add xfail coverage for control-mode gaps

why: Document current control-mode discrepancies and protect against regressions.
what:
- Add xfail regression tests for trailing stdout blanks, kill-server EOF, is_alive bootstrap, switch_client client counting
- Capture fixtures using NamedTuple parametrization with test_id identifiers
- Keep failing cases isolated while keeping lint/format/type checks passing

Author: tony

tests/test_control_mode_regressions.py

@@ -0,0 +1,141 @@
+"""Regression repros for current control-mode gaps (marked xfail)."""
+
+from __future__ import annotations
+
+import contextlib
+import typing as t
+import uuid
+
+import pytest
+
+from libtmux import exc
+from libtmux._internal.engines.base import ExitStatus
+from libtmux._internal.engines.control_mode import ControlModeEngine
+from libtmux._internal.engines.control_protocol import (
+    CommandContext,
+    ControlProtocol,
+)
+from libtmux.server import Server
+
+
+class TrailingOutputFixture(t.NamedTuple):
+    """Fixture for trailing-blank stdout normalization."""
+
+    test_id: str
+    raw_lines: list[str]
+    expected_stdout: list[str]
+
+
+TRAILING_OUTPUT_CASES = [
+    pytest.param(
+        TrailingOutputFixture(
+            test_id="no_blanks",
+            raw_lines=["line1"],
+            expected_stdout=["line1"],
+        ),
+        id="no_blanks",
+    ),
+    pytest.param(
+        TrailingOutputFixture(
+            test_id="one_blank",
+            raw_lines=["line1", ""],
+            expected_stdout=["line1"],
+        ),
+        id="one_blank",
+        marks=pytest.mark.xfail(
+            reason="control-mode preserves trailing blank stdout lines",
+            strict=True,
+        ),
+    ),
+    pytest.param(
+        TrailingOutputFixture(
+            test_id="many_blanks",
+            raw_lines=["line1", "", "", ""],
+            expected_stdout=["line1"],
+        ),
+        id="many_blanks",
+        marks=pytest.mark.xfail(
+            reason="control-mode preserves trailing blank stdout lines",
+            strict=True,
+        ),
+    ),
+]
+
+
+@pytest.mark.parametrize(
+    "case",
+    TRAILING_OUTPUT_CASES,
+)
+def test_control_protocol_trims_trailing_blank_lines(
+    case: TrailingOutputFixture,
+) -> None:
+    """ControlProtocol should trim trailing blank lines like subprocess engine."""
+    proto = ControlProtocol()
+    ctx = CommandContext(argv=["tmux", "list-sessions"])
+    proto.register_command(ctx)
+    proto.feed_line("%begin 0 1 0")
+    for line in case.raw_lines:
+        proto.feed_line(line)
+    proto.feed_line("%end 0 1 0")
+
+    assert ctx.done.wait(timeout=0.05)
+    result = proto.build_result(ctx)
+    assert result.stdout == case.expected_stdout
+
+
+@pytest.mark.xfail(
+    reason="kill-server EOF currently surfaces as ControlModeConnectionError",
+    strict=True,
+)
+def test_kill_server_eof_marks_success() -> None:
+    """EOF during kill-server should be treated as a successful completion."""
+    proto = ControlProtocol()
+    ctx = CommandContext(argv=["tmux", "kill-server"])
+    proto.register_command(ctx)
+    proto.feed_line("%begin 0 1 0")
+
+    proto.mark_dead("EOF from tmux")
+
+    assert ctx.done.is_set()
+    assert ctx.error is None
+    result = proto.build_result(ctx)
+    assert result.exit_status is ExitStatus.OK
+
+
+@pytest.mark.xfail(
+    reason="control-mode bootstrap currently makes is_alive start a server",
+    strict=True,
+)
+def test_is_alive_does_not_bootstrap_control_mode() -> None:
+    """is_alive should not spin up control-mode process for an unknown socket."""
+    socket_name = f"libtmux_test_{uuid.uuid4().hex[:8]}"
+    engine = ControlModeEngine()
+    server = Server(socket_name=socket_name, engine=engine)
+    try:
+        assert server.is_alive() is False
+        assert engine.process is None
+    finally:
+        # Best-effort cleanup; current behavior may have started tmux.
+        with contextlib.suppress(Exception):
+            server.kill()
+
+
+@pytest.mark.xfail(
+    reason="control-mode client counts as attached, so switch_client succeeds",
+    strict=True,
+)
+def test_switch_client_raises_without_user_clients() -> None:
+    """switch_client should raise when no user clients are attached."""
+    socket_name = f"libtmux_test_{uuid.uuid4().hex[:8]}"
+    engine = ControlModeEngine()
+    server = Server(socket_name=socket_name, engine=engine)
+
+    try:
+        session = server.new_session(session_name="switch_client_repro", attach=False)
+        assert session is not None
+
+        with pytest.raises(exc.LibTmuxException):
+            server.switch_client("switch_client_repro")
+    finally:
+        with contextlib.suppress(Exception):
+            server.kill()