fix: set IUserSession user after bearer token validation

[hangerrits]

Summary

When user_oidc validates a bearer token in Backend::getCurrentUserId(), it returns the correct userId but does not call IUserSession::setUser().
This leaves the user session in an inconsistent state where getCurrentUserId() succeeds but DI-injected $userId parameters remain null.

Problem

OCS controllers and CalDAV plugins that receive $userId via dependency injection get null instead of the authenticated user's ID when the request
is authenticated via OIDC bearer token. This causes:

• Deck: TypeError: ...$userId must be of type string, null given
• Talk: Same TypeError pattern
• Tasks (CalDAV): 500 errors from null userId

These apps work correctly with session-based OIDC login (where setUser() IS called) but fail with bearer token authentication.

Fix

Call IUserSession::setUser() after successful bearer token validation at all three return points in getCurrentUserId(). IUserSession is resolved via
Server::get() rather than constructor injection to avoid a circular dependency.

Testing

1. Configure an OIDC provider with bearer token validation enabled
2. Make API requests to Deck, Talk, or CalDAV endpoints using a bearer token
3. Verify 200 responses instead of 500 errors

[julien-nc]

LGTM. Thanks.
If you can rebase on main and fix the cs issue (run composer run cs:fix), let's merge that soon.