refactor-state-errors

What problem does it solve?

Refactor a service crate's state module to use domain-specific error enums instead of constructing end-user-facing error structs (error_type/message/status) and move error shaping into the handler layer.

Core Features & Use Cases

• Introduces a domain-focused error enum using thiserror to represent service errors without HTTP/wire knowledge.
• Replaces all {Service}Error { error_type: ..., message: ..., status: ... } constructions in state.rs with explicit enum variants.
• Updates the handler error-shaping function to exhaustively map enum variants to wire responses, while using err.to_string() for the message to keep a single source of truth.

Quick Start

Replace end-user error structs in state.rs with a domain-specific error enum and move domain-to-wire error mapping into handlers.rs.