Proposed changes

Problem

When a local Rocket.Chat user self-joins a federated room via channels.join (or through the UI "Join" button), they are not registered on the Matrix homeserver. This means:

• The user cannot send messages that appear on the federated side
• The user's presence is invisible to Matrix participants
• The room membership state is inconsistent between RC and Matrix

Root Cause

The existing beforeAddUserToRoom hook at ee/server/hooks/federation/index.ts:105-139 requires an inviter parameter:

// Line 107 — early return when no inviter
if (!user.username || !inviter) {
    return;
}

When a user self-joins via channels.join, addUserToRoom is called without an inviter, so the federation hook silently exits and the Matrix bridge is never notified.

Users who are explicitly invited (via addUserToRoom with an inviter) work correctly because the inviter parameter is present.

Fix

Added an afterJoinRoom callback that catches the self-join case:

1. Verifies the room is federated (shouldPerformFederationAction
2. Skips external Matrix users (already handled by incoming federation events)
3. Calls FederationMatrix.inviteUsersToRoom() to register the local user on the Matrix homeserver
4. Uses LOW priority to run after all other join hooks
5. Wrapped in try/catch to avoid breaking the join flow if the Matrix call fails

Note on duplicate invites

For explicitly invited users, both beforeAddUserToRoom (existing) and this new afterJoinRoom hook will fire. The Matrix protocol handles duplicate invite events idempotently (re-inviting an already-joined member is a no-op), so this should be safe. However, I'd appreciate confirmation on this behavior. An alternative approach would be to use a module-scoped flag to track whether beforeAddUserToRoom already handled the invite for this user.

Issue(s)

Fixes #38239 Builds on the failing e2e test from #38669

@ggazzo Would love your feedback on the approach, especially regarding the duplicate invite edge case for explicitly invited users.