Improve typing for recv/send_metadata

Author: wence-

python/rapidsmpf/rapidsmpf/streaming/core/channel.pyi

@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: Copyright (c) 2025, NVIDIA CORPORATION & AFFILIATES.
+# SPDX-FileCopyrightText: Copyright (c) 2025-2026, NVIDIA CORPORATION & AFFILIATES.
 # SPDX-License-Identifier: Apache-2.0
 
 from __future__ import annotations
@@ -17,6 +17,6 @@ class Channel(Generic[PayloadT]):
     async def shutdown(self, ctx: Context) -> None: ...
     async def shutdown_metadata(self, ctx: Context) -> None: ...
     async def send(self, ctx: Context, item: Message[PayloadT]) -> None: ...
-    async def send_metadata(self, ctx: Context, item: Message[PayloadT]) -> None: ...
+    async def send_metadata(self, ctx: Context, item: Message) -> None: ...
     async def recv(self, ctx: Context) -> Message[PayloadT] | None: ...
-    async def recv_metadata(self, ctx: Context) -> Message[PayloadT] | None: ...
+    async def recv_metadata(self, ctx: Context) -> Message | None: ...

python/rapidsmpf/rapidsmpf/streaming/cudf/channel_metadata.pyi

@@ -4,7 +4,7 @@
 
 from __future__ import annotations
 
-from typing import Literal
+from typing import Literal, Self
 
 from rapidsmpf.streaming.core.message import Message
 
@@ -40,9 +40,9 @@ class ChannelMetadata:
         partitioning: Partitioning | None = None,
         duplicated: bool = False,
     ) -> None: ...
-    @staticmethod
-    def from_message(message: Message) -> ChannelMetadata: ...
-    def into_message(self, sequence_number: int, message: Message) -> None: ...
+    @classmethod
+    def from_message(cls: type[Self], message: Message[Self]) -> ChannelMetadata: ...
+    def into_message(self, sequence_number: int, message: Message[Self]) -> None: ...
     @property
     def local_count(self) -> int: ...
     @property