Server(fix[confirm_before,command_prompt]): gate -b behind has_gte_version("3.3")

why: Both methods unconditionally emit -b despite their docstrings noting it
     requires tmux 3.3+. On older tmux this causes a hard command error instead
     of the project-convention warn-and-skip behaviour.
what:
- confirm_before: add has_gte_version("3.3") guard; warn and skip -b on older tmux
- command_prompt: same
- Add lazy imports for warnings and has_gte_version inside each method,
  matching the pattern already used in pane.py

Author: tony

src/libtmux/server.py

@@ -866,7 +866,19 @@ def confirm_before(
         ...     server.cmd('show-options', '-gv', '@cf_test').stdout[0]
         'yes'
         """
-        tmux_args: tuple[str, ...] = ("-b",)
+        import warnings
+
+        from libtmux.common import has_gte_version
+
+        tmux_args: tuple[str, ...] = ()
+
+        if has_gte_version("3.3", tmux_bin=self.tmux_bin):
+            tmux_args += ("-b",)
+        else:
+            warnings.warn(
+                "confirm_before -b requires tmux 3.3+, ignoring",
+                stacklevel=2,
+            )
 
         if prompt is not None:
             tmux_args += ("-p", prompt)
@@ -941,7 +953,19 @@ def command_prompt(
         ...     server.cmd('show-options', '-gv', '@cp_test').stdout[0]
         'hi'
         """
-        tmux_args: tuple[str, ...] = ("-b",)
+        import warnings
+
+        from libtmux.common import has_gte_version
+
+        tmux_args: tuple[str, ...] = ()
+
+        if has_gte_version("3.3", tmux_bin=self.tmux_bin):
+            tmux_args += ("-b",)
+        else:
+            warnings.warn(
+                "command_prompt -b requires tmux 3.3+, ignoring",
+                stacklevel=2,
+            )
 
         if one_key:
             tmux_args += ("-1",)