Core types and error definitions for the Argentor framework.

This crate provides the foundational types shared across all Argentor crates, including error handling, message representations, and tool call abstractions.

Main types

• [ArgentorError] — Unified error enum for all Argentor subsystems.
• [ArgentorResult] — Convenience alias for Result<T, ArgentorError>.
• [Role] — Message role (user, assistant, system, tool).
• [Message] — A single message within a conversation session.
• [ToolCall] — Represents an LLM-initiated tool invocation request.
• [ToolResult] — The result returned after executing a tool call.