fix(collaboration): allow inactive users to be added as collaborators and team members

[lunny]

Summary

Inactive users should remain unable to sign in, but otherwise be treated the same as regular users for collaboration and membership operations.

This PR removes the inactive-user restriction from collaborator and membership-related flows, so inactive users can still be added to repositories, teams, and other similar relationships.

Changes

• allow inactive users to be added as repository collaborators via Web UI
• allow inactive users to be added as repository collaborators via API
• include inactive users in candidate user search results used by collaboration/member pickers
• add regression tests for:
  • adding an inactive user as a collaborator
  • adding an inactive user as a team member
  • searching inactive users in collaboration candidate search

Behavior

Before:

• inactive users could not be selected or added in some collaboration/member flows

After:

• inactive users still cannot log in
• inactive users can be added to collaborators, teams, and similar membership relationships like normal users

[bircni]

Is it then visible that the specific user is inactive?