Fixed: Validation error messages now return field paths

Request validation errors now include the full field path in the error message, making it easier to identify which nested field caused a validation failure.

Before:

{ "error": "Invalid input" }

After:

{
  "error": "Validation failed",
  "details": [
    { "field": "applicant.address.postal_code", "message": "Invalid postal code format" }
  ]
}