internal/subprocess(refactor[typing]): Align preexec_fn with typeshed

why: Use Callable[[], object] to match Python's official typeshed,
accepting functions that return values (which subprocess ignores).
what:
- Change preexec_fn return type from None to object
- Update both subprocess.py and run.py

Author: tony

src/libvcs/_internal/run.py

@@ -130,7 +130,7 @@ def run(
     stdin: _FILE | None = None,
     stdout: _FILE | None = None,
     stderr: _FILE | None = None,
-    preexec_fn: t.Callable[[], None] | None = None,
+    preexec_fn: t.Callable[[], object] | None = None,
     close_fds: bool = True,
     shell: bool = False,
     cwd: StrOrBytesPath | None = None,

src/libvcs/_internal/subprocess.py

@@ -95,7 +95,7 @@ class SubprocessCommand(SkipDefaultFieldsReprMixin):
     stdin: _FILE = None
     stdout: _FILE = None
     stderr: _FILE = None
-    preexec_fn: t.Callable[[], None] | None = None
+    preexec_fn: t.Callable[[], object] | None = None
     close_fds: bool = True
     shell: bool = False
     cwd: StrOrBytesPath | None = None