#41689 fix(federation): error on first user interaction

Proposed changes (including videos or screenshots)

First federation user interaction was failing due to createOrUpdateFederatedUser was returning a projection with less fields than needed. So now createOrUpdateFederatedUser correctly returns a full IUser and new tests were added to validate that.

Issue(s)

CORE-2495

Steps to test or reproduce

Further comments

Summary by CodeRabbit

• New Features

  • Improved federated user resolution and automatic creation during Matrix invitations, joins, and first-contact interactions.
  • Added validation for federated usernames, with clear errors for invalid IDs.
  • Improved direct-message room naming when display names are unavailable.

• Bug Fixes

  • Prevented local users and invalid federated results from being treated as remote users.
  • Improved error handling while preserving underlying causes.

• Tests

  • Added coverage for user resolution, registration validation, and first-contact federation flows.