feat: restore federation metrics lost in native federation migration

[KevLehman]

Proposed changes (including videos or screenshots)

Restores the federation statistics that regressed in the migration from the old federation implementations to native federation. Three claims fixed:

1. externalConnectedServers always reported {quantity: 0, servers: []}. Its backing query, Rooms.countDistinctFederationRoomsExcluding, was a // TODO implement stub returning []. It now counts remote homeservers from rocketchat_federation_servers (the federation-sdk signing-key cache — one doc per remote homeserver we verified), excluding Federation_Service_Domain. The dead stub and its IRoomsModel typing are removed.

2. Federation event volume was dropped entirely. The old federation reported federationOverviewData.numberOfEvents (lifetime event-store count); the native stack persists a full PDU store in rocketchat_federation_events but never reported it. New amountOfFederationEvents field on matrixFederation statistics, fed by estimatedDocumentCount() (collection metadata, no scan).

3. amountOfExternalUsers over-counted on upgraded workspaces. Users.countFederatedExternalUsers counted {federated: true}, which includes stale users created by the removed matrix bridge (federated: true with no federation subdoc — verified against 6.9.7's FederatedUser.getStorageRepresentation). The query now also requires federation.version, which only native federation writes. LDAP-provisioned federated users keep being counted: they get the full native subdoc and are genuinely remote (getFederationHomeServer only fires when the mapped homeserver differs from the local domain).

Missing-collection safety: distinct/estimatedDocumentCount on nonexistent collections return []/0, so workspaces that never federated report zeros.

Issue(s)

Follow-up to #37059 and #41940 (old federation removals).

Steps to test or reproduce

1. Run two federated workspaces (native federation enabled) and exchange messages in a federated room.
2. Call POST /api/v1/statistics.generate (or wait for the usage report cron) on one of them.
3. matrixFederation.externalConnectedServers lists the peer homeserver, amountOfFederationEvents > 0, and amountOfExternalUsers counts only users with a native federation subdoc.

Further comments

The raw db.collection() reads in the statistics adapter are deliberate: rocketchat_federation_servers and rocketchat_federation_events are owned by @rocket.chat/federation-sdk (no model exists on the Rocket.Chat side), and creating models for two read-only counts wasn't worth the ceremony.

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-9.0.0@2b359ff). Learn more about missing BASE report.

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