refactor: Improve DAP server code organisation and readability

[JoshuaBatty]

Description

This PR improves the organization and readability of the forc-debug codebase through several changes:

• Moves CLI functionality into a dedicated CLI module
• Creates a unified error handling system using thiserror
• Improves command handler organization and readability
• Extracts common functionality into reusable helper methods
• Adds clear documentation for public interfaces
• Introduces the HandlerResult type to simplify DAP server response handling
• Implements consistent error propagation throughout the codebase
• Ran clippy pedantic

Improvements:

• Better separation of concerns between CLI and server code
• More descriptive error messages with proper context
• Cleaner command handling

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[sdankel]

Thanks for improving the error handling. The changes look good!

[tritao]

Good stuff 👍