Refactor Opportunity validation to use custom exceptions

[sonalguptaabd-bit]

Summary

Opportunity validation currently uses addError directly inside the validation
service, which mixes validation logic with user-facing error handling.

Suggested Solution

Refactor Opportunity validation to throw custom exception classes
(e.g., OpportunityValidationException) from the validation service.

Catch these exceptions in the OpportunityTriggerHandler and surface
user-friendly messages using addError, improving separation of concerns
and aligning with the handler/service pattern used in this repository.

Alternative Solutions

Continue using addError directly in the validation service, but this would
keep business validation logic tightly coupled with UI error handling.

Code of Conduct

• I agree to follow this project's Code of Conduct

[github-actions]

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[msrivastav13]

This looks good to me. If you have implemented this in a pull request, please link the issue with the pull request for further reviews.

[sonalguptaabd-bit]

Thanks for the review!
Yes, I’ve implemented this change in a pull request.

PR link: #691

Please let me know if any changes are needed.