Per-session MFA for desktops is broken

[zmb3]

When attempting to connect to a desktop where per-session MFA is required, I am correctly prompted for an MFA, but after tapping the webauthn device I never see the remote desktop - the UI just remains at the spinner.

Browser dev tools show the MFA challenge going out over websocket:

Nothing super interesting in the Teleport logs (I suspect this is a web UI issue):

2024-12-23T11:18:59.639-07:00 DEBU [WEB]       Attempting to connect to desktop user:zac session:73d7 desktop_name:dynamic-standalone-2019 cluster_name:zac-local username:Administrator web/desktop.go:109
2024-12-23T11:18:59.639-07:00 DEBU [WEB]       Attempting to connect to desktop user:zac session:73d7 desktop_name:dynamic-standalone-2019 cluster_name:zac-local username:Administrator width:1497 height:1291 web/desktop.go:130
2024-12-23T11:18:59.644-07:00 INFO  emitting audit event event_type:mfa_auth_challenge.create fields:map[challenge_allow_reuse:false challenge_scope:CHALLENGE_SCOPE_USER_SESSION cluster_name:zac-local code:T1015I ei:0 event:mfa_auth_challenge.create time:2024-12-23T18:18:59.644Z trace.component:audit uid:dc40a423-a865-4ce2-96d4-1abdeb31faa2 user:zac user_kind:1] events/emitter.go:287
2024-12-23T11:19:51.575-07:00 DEBU [WEB]       Received non-MFA message, withholding msg_type:tdp.ClientScreenSpec trace_id:b08dbd3cdc5bbde818c7c12e2d9e5fed span_id:81934a26473a88cd web/desktop.go:424
2024-12-23T11:19:58.096-07:00 ERRO [WEB]       creating desktop connection failed user:zac session:73d7 desktop_name:dynamic-standalone-2019 cluster_name:zac-local error:[websocket: close sent] web/desktop.go:79

Questions:

• why do we see two "attempting to connect to desktop" messages?

I confirmed that this brokenb behavior is present on master (d070ce0), but is not present on branch/v17 (909aca2), which suggests that #49794 (comment) is probably the culprit.