Fix typo in DomainError class definition

[ShivenSiwach]

What does this PR do?

Closes #

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Documentation
• Infrastructure / CI
• Test coverage

Checklist

• I have read CONTRIBUTING.md
• My code follows the project's coding standards (docs/CODING-RULES.md)
• I have added tests for new functionality
• All existing tests pass locally (python -m pytest tools/arch-test/ -v)
• I have updated documentation if my change affects APIs, config, or architecture
• No file exceeds 300 lines
• No function exceeds 50 lines
• No route handler exceeds 80 lines

Architecture compliance

• No cross-service imports (each service owns its own schema)
• No new services created without discussion
• Event contracts follow the outbox pattern
• API changes are backward-compatible or documented as breaking