Add AssistantMessageError type (#352)

This brings Python to parity with
anthropics/claude-cli-internal#10358.

Author: shawnm-anthropic

src/claude_agent_sdk/types.py

@@ -457,6 +457,16 @@ class ToolResultBlock:
 
 
 # Message types
+AssistantMessageError = Literal[
+    "authentication_failed",
+    "billing_error",
+    "rate_limit",
+    "invalid_request",
+    "server_error",
+    "unknown",
+]
+
+
 @dataclass
 class UserMessage:
     """User message."""
@@ -472,6 +482,7 @@ class AssistantMessage:
     content: list[ContentBlock]
     model: str
     parent_tool_use_id: str | None = None
+    error: AssistantMessageError | None = None
 
 
 @dataclass