Replace 7 individual ToolResultEvent type guards (isBashToolResult,
isReadToolResult, etc.) with a unified isToolResultEventType() function,
mirroring the existing isToolCallEventType() pattern.
Inline 14 handler type aliases (SendMessageHandler, SetModelHandler, etc.)
directly into the ExtensionActions interface since they were only used there
and added no semantic value.
Update documentation examples to use the new unified guard.
