refactor(channel): centralize retry around explicit send failures

Make channel delivery failures raise consistently so retry policy lives in ChannelManager rather than being split across individual channels. Tighten Telegram stream finalization, clarify sendMaxRetries semantics, and align the docs with the behavior the system actually guarantees.

nanobot/channels/base.py

@@ -85,11 +85,18 @@ class BaseChannel(ABC):
 
         Args:
             msg: The message to send.
+
+        Implementations should raise on delivery failure so the channel manager
+        can apply any retry policy in one place.
         """
         pass
 
     async def send_delta(self, chat_id: str, delta: str, metadata: dict[str, Any] | None = None) -> None:
-        """Deliver a streaming text chunk. Override in subclass to enable streaming."""
+        """Deliver a streaming text chunk.
+
+        Override in subclasses to enable streaming. Implementations should
+        raise on delivery failure so the channel manager can retry.
+        """
         pass
 
     @property