fix(auth): stop beforeAuthStateChanged from holding the app unstable

[armando-navarro]

Fixes #3748

Restores beforeAuthStateChanged: { blockUntilFirst: false }, which #3590 added and #3613 dropped without comment.

Changes

• @angular/fire/auth wraps beforeAuthStateChanged with blockUntilFirst set to true, so registering the hook adds a pending task that clears only when the callback first runs. Firebase runs this callback only on a sign-in or sign-out, so for a visitor who does neither, the app never becomes stable.
  • In the browser, ApplicationRef.whenStable() never resolves.
  • When the hook is also registered on the server, ng build fails during route extraction, and a server that renders per request never responds.
• With the override, the callback still runs inside Angular's zone and injection context, and its returned promise still reaches Firebase, so a rejection still cancels the sign-in.
  • One other difference: a call outside an injection context now logs its per-call warning only at the verbose log level, as onMessage does.
• Adding more injector safety #3590 added the override with the note "beforeAuthStateChanged should not block". zone log verbosity #3613 removed it at the spot where it added log-level overrides, and none of the issues zone log verbosity #3613 fixed involve this function, so the removal looks accidental.
• docs/auth.md and the sample keep importing beforeAuthStateChanged from firebase/auth until a release carries this fix.

Verification

On an Angular 21.2 server-rendered app, comparing 21.0.0-rc.1 with a build of this branch:

• Hook registered on the server: ng build on rc.1 fails after 34 seconds with Routes extraction was aborted. TimeoutError. With this branch it succeeds.
• Hook registered on the server only while answering a request, with every route rendered per request: each request to rc.1 got no response within 60 seconds. With this branch, each returned a server-rendered page in under 30 milliseconds.
• The cookie sync from docs/auth.md, which registers the hook only in the browser: on rc.1 the app did not become stable within 10 seconds. With this branch it became stable after 25 milliseconds.
• npm run test:node: 331 specs, 0 failures. npm run test:chrome-headless: 107 passed, 48 skipped.