Add helper for building a standardized 422 validation error response shape

[Chucks1093]

Summary

Validation errors (invalid params, exceeded limits, malformed input) are returned with inconsistent shapes across different endpoints. A shared helper that builds a standardized 422 response body should be used everywhere a validation error is returned.

Scope

• Add buildValidationError(field: string, message: string, code: string): ValidationErrorResponse helper
• Response shape: { error: { code, field, message } }
• Add unit tests confirming the shape is correct for several different field and message combinations

Acceptance Criteria

• Helper returns correct shape with all three fields
• Used consistently across all 422 responses in the codebase
• Unit tests cover multiple field and message combinations

ETA: 12 hours

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[dorisadams]

I would love to work on this issue, please assign the issues to me so i Can work on it ASAP

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[villadel]

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