fix(typing): Include Expr in fill_null signature (#2129)

Follow-up to (#2106)
Resolves https://github.com/narwhals-dev/narwhals/pull/2106/files#r1976489510

Author: dangotbanned

narwhals/_arrow/expr.py

@@ -361,7 +361,7 @@ def sample(
 
     def fill_null(
         self: Self,
-        value: Any | None,
+        value: Self | Any | None,
         strategy: Literal["forward", "backward"] | None,
         limit: int | None,
     ) -> Self:

narwhals/_dask/expr.py

@@ -406,7 +406,7 @@ def any(self: Self) -> Self:
 
     def fill_null(
         self: Self,
-        value: Any | None,
+        value: Self | Any | None,
         strategy: Literal["forward", "backward"] | None,
         limit: int | None,
     ) -> DaskExpr:

narwhals/_duckdb/expr.py

@@ -445,7 +445,9 @@ def round(self: Self, decimals: int) -> Self:
             lambda _input: FunctionExpression("round", _input, lit(decimals)), "round"
         )
 
-    def fill_null(self: Self, value: Any, strategy: Any, limit: int | None) -> Self:
+    def fill_null(
+        self: Self, value: Self | Any, strategy: Any, limit: int | None
+    ) -> Self:
         if strategy is not None:
             msg = "todo"
             raise NotImplementedError(msg)

narwhals/_pandas_like/expr.py

@@ -315,7 +315,7 @@ def is_nan(self: Self) -> Self:
 
     def fill_null(
         self: Self,
-        value: Any | None,
+        value: Self | Any | None,
         strategy: Literal["forward", "backward"] | None,
         limit: int | None,
     ) -> Self:

narwhals/expr.py

@@ -1327,7 +1327,7 @@ def arg_true(self: Self) -> Self:
 
     def fill_null(
         self: Self,
-        value: Any | None = None,
+        value: Expr | Any | None = None,
         strategy: Literal["forward", "backward"] | None = None,
         limit: int | None = None,
     ) -> Self: