feat(users): add phone number support to user search

Description

This merge request extends GET /api/v1/users/{user_identifier} to support searching users by phone number in addition to existing username and userId lookup.

Changes

• Added phoneNumber as a supported search criterion
• Implemented phone number normalization before lookup
• Updated API documentation
• Preserved existing username and userId search behavior

Expected Behavior

• Users can be searched using full phone numbers
• Existing username and userId searches continue to work unchanged
• Normalized phone number matching is handled consistently

Impact

• Improves user discoverability via phone number
• Existing functionality remains unaffected
• Considered privacy implications related to phone number search

Checklist

• The feature has been fully implemented.
• Tests for the new feature are included and passing.
• User documentation/guides have been updated (if applicable).
• Impact on existing functionality has been considered.

Related Issue(s)

Closes #130 (closed)