Constant MAX_STATE_KEY_LEN 

pub const MAX_STATE_KEY_LEN: usize = 256; // 256usize

Core traits and types.

Always available regardless of feature flags. Includes:

• Agent - The fundamental trait for all agents
• Tool / Toolset - For extending agents with capabilities
• Session / State - For managing conversation context
• Event - For streaming agent responses
• AdkError / Result - Unified error handling Maximum allowed length for state keys (256 bytes).