FINERACT-405: Standardize mobile number validation across Client and …

[codigm]

Description

Resolves FINERACT-405.

This PR standardizes mobile number validation across the Client and Staff modules.
Previously, the regular expression ^\+?[0-9]{7,15}$ was duplicated across three different files (ClientDataValidator, StaffCreateRequest, and StaffUpdateRequest).

This PR extracts the regex into a centralized ValidationConstants.java utility class in fineract-core and updates all references to use this single source of truth, improving maintainability.

Checklist

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• Add required Swagger annotation and update API documentation (Note: Not applicable, no API signatures changed)
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[AshharAhmadKhan]

@codigm please check the failures. Also another PR #5964 tried fixing the same issue, have a look at that as well for pointers.