Consolidating Diagnostic Formatting Responsibilities

[snowykr]

• Current state: Diagnostic formatting is scattered across three locations:
  • Diagnostic.__str__
  • CompileResult.format_diagnostics
  • CheckResult.format_diagnostics
  • _format_diagnostics_with_label (in runner)
• Long-term goal:
  • Implement a "common formatter" utility in the diagnostics module (e.g., format_diagnostics(diagnostics: list[Diagnostic])).
  • Separate responsibilities so that the runner focuses solely on "how to prepend labels," while the common utility handles the core formatting.