feat(#11129): support filtering users by oidc_username

[santhosh-7777]

Description

Adds support for finding users by oidc_username through the existing users API.
This allows integrations to retrieve users using requests such as:
GET /api/v2/users?oidc_username=cha@registry.com
The implementation follows the existing filtering pattern used for facility_id and contact_id.

Fixes #11129

Code review checklist

• Readable: Concise, well named, follows the style guide
• Tested: Added controller and user-management unit tests
• Backwards compatible: Extends existing filtering functionality without changing existing behavior

Testing

• Added controller tests for oidc_username query parameter handling.
• Added user-management service tests for filtering users by oidc_username.
• Verified existing filtering behavior remains unchanged.

[santhosh-7777]

@andrablaj hii could u please review this ?