lib.FileContext: Interpolate path in _is_dir()

_is_dir() logs a DEBUG message when _stat() is not implemented and it
must guess from the trailing slash whether a path is a directory. The
second part of that message is a plain string rather than an f-string,
so '{path}' is logged verbatim instead of the path.

Make it an f-string so the path is interpolated.

Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.84.2
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/FileContext.py

@@ -287,7 +287,7 @@ class FileContext(abc.ABC):
                 DEBUG,
                 (
                     f"{self.log_name} doesn't implement stat(), judging by trailing "
-                    'slash if {path} is a directory'
+                    f'slash if {path} is a directory'
                 ),
             )
             return path[-1] == '/'