Add usage field to AssistantMessage

The CLI returns per-turn usage data on each assistant message but the SDK
was dropping it during parsing. This exposes it as an optional dict on
AssistantMessage so consumers can track context token usage mid-stream.

Author: Mng-dev-ai

src/claude_agent_sdk/_internal/message_parser.py

@@ -127,6 +127,7 @@ def parse_message(data: dict[str, Any]) -> Message:
                     model=data["message"]["model"],
                     parent_tool_use_id=data.get("parent_tool_use_id"),
                     error=data.get("error"),
+                    usage=data.get("message", {}).get("usage"),
                 )
             except KeyError as e:
                 raise MessageParseError(

src/claude_agent_sdk/types.py

@@ -657,6 +657,7 @@ class AssistantMessage:
     model: str
     parent_tool_use_id: str | None = None
     error: AssistantMessageError | None = None
+    usage: dict[str, Any] | None = None
 
 
 @dataclass