fix: use Self return type for SCMContext.__enter__ and __call__

Return self directly in __enter__ instead of the @contextmanager's
__enter__() return value. Also update __call__ to return
Iterator[Self]. They're semantically identical (the contextmanager
yields self), but this satisfies both ruff PYI034 and mypy.

https://claude.ai/code/session_01M18n2k6KLenwMRANgSFMXc

dvc/repo/scm_context.py

@@ -98,7 +98,7 @@ class SCMContext:
     @contextmanager
     def __call__(
         self, autostage: Optional[bool] = None, quiet: Optional[bool] = None
-    ) -> Iterator["SCMContext"]:
+    ) -> Iterator["Self"]:
         try:
             yield self
         except Exception:
@@ -135,7 +135,8 @@ class SCMContext:
 
     def __enter__(self) -> "Self":
         self._cm = self()
-        return self._cm.__enter__()
+        self._cm.__enter__()
+        return self
 
     def __exit__(self, *exc_args) -> None:
         assert self._cm