[FEATURE] Add PATCH endpoint for partial user updates

Problem

The current PUT /api/v1/users endpoint requires all mandatory fields (userId, firstName, lastName, email) even when updating a single field. This creates unnecessary complexity for API consumers.
Current behavior:
PUT /api/v1/users { "userId": "dotcms.org.2806", "firstName": "John", // Required even if not changing "lastName": "Doe", // Required even if not changing "email": "new@email.com" // The only field being updated }

Goal

Proposed Solution
Add a PATCH /api/v1/users endpoint that supports partial updates, allowing users to update only specific fields.

New behavior:
PATCH /api/v1/users{   
 "userId": "dotcms.org.2806",   
 "email": "new@email.com"   // Only send what's changing
}

Target Personas

• Developer teams
• Content teams
• DevOps teams
• System administrators (dotCMS)

Demo Expectations

Able to use update user

Acceptance Criteria

• PATCH endpoint accepts userId + any optional user fields
  -Unspecified fields retain existing values
• Email, firstName, lastName, and password can each be updated independently

User Stories

No response

Links

https://helpdesk.dotcms.com/a/tickets/33011

Start by locating the existing PUT /api/v1/users implementation and its user update tests. Trace how required fields are validated and persisted, then add the PATCH behavior described in the acceptance criteria and verify that omitted fields retain their existing values while email, firstName, lastName, and password can be updated independently.