feat: accurate source locations for schema diagnostics (#171)

[pedrosakuma]

Summary

• capture XML element/attribute source locations during schema parsing and carry them through DTOs
• report schema diagnostics against the relevant XML location when available, including parse-time and downstream generation-time validation paths
• document the new behavior and add end-to-end tests covering missing attributes, invalid integer attributes, and sinceVersion diagnostics

Validation

• dotnet build
  • Build succeeded
  • 0 Warning(s)
  • 0 Error(s)
• dotnet test
  • SbeCodeGenerator.Tests: Passed 196/196
  • SbeCodeGenerator.IntegrationTests: Passed 168/168

Closes #171