Discussion creation from `pmid` auto invites deactivated message authors

Description

When creating a discussion from a message (rooms.createDiscussion with pmid), the original message author is automatically included in the invite list.
Currently, this auto-invite path does not check whether that user is deactivated.

Current behavior

In apps/meteor/app/discussion/server/methods/createDiscussion.ts, the invite list is built as:

• message ? [message.u.username, ...users] : users

This means the message author is included even if active: false.

Expected behavior

Deactivated users should not be auto invited when creating discussions from a message.
If a username is deactivated, it should be filtered out before room creation.

Why this matters

• Keeps membership behavior consistent with “active users only” expectations.
• Avoids attempts to include inactive accounts in new discussion participants.
• Reduces edge case permission/subscription inconsistencies.

Minimal reproduction

1. Create user u1 and ensure they can post in a channel.
2. u1 sends a message in channel C.
3. Deactivate u1 (users.setActiveStatus, activeStatus: false).
4. Another active user creates a discussion from u1’s message (pmid set).
5. Observe that u1 is currently considered for auto-invite.

Suggested fix

In discussion creation flow:

1. Build candidate usernames (message author + users[]).
2. Resolve usernames to active users only.
3. Pass only active usernames to createRoom.

Acceptance criteria

• Deactivated message authors are not auto-invited to newly created discussions.
• Deactivated usernames in users[] are ignored.
• Add an end-to-end test under /rooms.createDiscussion covering this case.

Start in apps/meteor/app/discussion/server/methods/createDiscussion.ts and trace the /rooms.createDiscussion flow for pmid and users. Add or update the end-to-end test under /rooms.createDiscussion to cover a deactivated message author and deactivated usernames in users[]. Done means inactive users are excluded from newly created discussion invites.