textframe(fix): Make TextFrameExtension import conditional

why: Users without libtmux[textframe] get ImportError on capture_frame()
what:
- Wrap TextFrameExtension import in try/except ImportError
- Only add to __all__ when syrupy is available
- Core TextFrame functionality works without optional dependency

Author: tony

src/libtmux/textframe/__init__.py

@@ -3,6 +3,13 @@
 from __future__ import annotations
 
 from libtmux.textframe.core import ContentOverflowError, TextFrame
-from libtmux.textframe.plugin import TextFrameExtension
 
-__all__ = ["ContentOverflowError", "TextFrame", "TextFrameExtension"]
+__all__ = ["ContentOverflowError", "TextFrame"]
+
+# Conditionally export TextFrameExtension when syrupy is available
+try:
+    from libtmux.textframe.plugin import TextFrameExtension
+
+    __all__.append("TextFrameExtension")
+except ImportError:
+    pass