tmux_cmd(fix[FileNotFoundError]): Raise TmuxCommandNotFound for invalid tmux_bin

why: Invalid custom tmux_bin raised raw FileNotFoundError from subprocess
instead of the domain-specific TmuxCommandNotFound, creating an
inconsistent API surface.
what:
- Catch FileNotFoundError before generic Exception in tmux_cmd.__init__
- Re-raise as TmuxCommandNotFound from None
- Update test_tmux_bin_invalid_path to expect TmuxCommandNotFound

Author: tony

src/libtmux/common.py

@@ -276,6 +276,8 @@ def __init__(self, *args: t.Any, tmux_bin: str | None = None) -> None:
             )
             stdout, stderr = self.process.communicate()
             returncode = self.process.returncode
+        except FileNotFoundError:
+            raise exc.TmuxCommandNotFound from None
         except Exception:
             logger.error(  # noqa: TRY400
                 "tmux subprocess failed",

tests/test_server.py

@@ -432,7 +432,9 @@ def test_tmux_bin_custom_path() -> None:
 
 
 def test_tmux_bin_invalid_path() -> None:
-    """Invalid tmux_bin raises on command execution."""
+    """Invalid tmux_bin raises TmuxCommandNotFound."""
+    from libtmux import exc
+
     s = Server(tmux_bin="/nonexistent/tmux")
-    with pytest.raises(FileNotFoundError):
+    with pytest.raises(exc.TmuxCommandNotFound):
         s.cmd("list-sessions")