Fix/error redirect meeting logouturl

[antobinary]

What does this PR do?

Makes API error redirects (e.g. guestDeniedAccess, maxParticipantsReached, mismatchCreateTimeParam) fall back to the logoutURL stored on the meeting at /create time, instead of jumping straight to the server default (serverURL).

The redirect target precedence is now:

1. errorRedirectUrl (join param)
2. logoutURL (join param)
3. logoutURL (create param, stored on the meeting) — new
4. bigbluebutton.web.logoutURL / serverURL (server default)

Closes bigbluebutton#23380

Motivation

The docs for errorRedirectUrl state that when no errorRedirectUrl is provided, users are redirected to "the meeting's default logout URL". However respondWithRedirect() only consulted the current request's logoutURL parameter — a logoutURL passed on /create was never considered, so integrations that set it once at create time had denied users land on serverURL.

Repro (before this change):

1. create with logoutURL=https://ddg.gg&guestPolicy=ALWAYS_DENY
2. join as a guest viewer with redirect=true
3. → 302 Location: https://<serverURL>?errors=[...guestDeniedAccess...] instead of https://ddg.gg?errors=[...]

Implementation notes

• invalid() / respondWithRedirect() accept a new optional meetingLogoutUrl argument; the meeting-scoped error paths in the join flow pass the effective logout URL that the join handler already computes (meeting value + join-param override + %%FIELDS%% token substitution).
• Unlike the join-time logoutURL param, the create-time value is not URL-validated by processLogoutUrl(), so the fallback is only used if it passes ValidationService.isValidURL() — a malformed value keeps today's behavior (server default) rather than producing a broken redirect.
• Error paths that fire before a meeting is resolved (checksum/validation errors) are unchanged.

Testing

Verified on a 3.0 dev container (v3.0.x-develop):

Scenario	Before	After
logoutURL on create only, guest denied	serverURL?errors=...	https://ddg.gg?errors=...
logoutURL on join (differs from create)	join value honored	join value still wins over create value
errorRedirectUrl on join	honored	honored (still wins over any logoutURL)
No logoutURL anywhere	serverURL?errors=...	serverURL?errors=... (unchanged)
Malformed logoutURL on create (notaurl)	n/a	falls back to serverURL?errors=..., no broken redirect

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