docs: fix broken code snippets in product guides

[armando-navarro]

Description

Fixes the broken copy-paste code samples in the modular product guides. Each change makes the sample compile or parse as written, so a developer can copy it directly.

• Close the appConfig object literal with } (was })) in the App Check, Auth, Database, Functions, Messaging, Remote Config, Storage, and Performance guides.
• Import ApplicationConfig in every appConfig sample so the type annotation resolves.
• App Check: import getApp, which the sample calls.
• Firestore: correct export Interface to export interface (twice), import Observable from rxjs, and import CollectionReference and DocumentReference.
• Auth and Database: replace the @NgModule emulator and multiple-instance samples with the standalone appConfig form the rest of the guides teach, and inline the Firebase config instead of reading environment.firebase, which the current ng add setup does not generate.
• Messaging: repair the FcmService example (declare message$, move its assignment into the constructor, make deleteToken async) and bump the service-worker CDN imports from 9.22.0 to 12.4.0 to match the firebase dependency.

Documentation only, no source or API changes.

Checklist

• Issue number for this PR: docs: broken code samples in several product guides #3731
• Docs included?: yes (documentation-only change)
• Test units included?: no (documentation-only change)
• In a clean directory, yarn install, yarn test run successfully?: not applicable, no code changed

Fixes #3731
Refs #3369

[tyler-reitz]

Nice cleanup — these are the samples people copy first, so having them parse matters.

I pulled the branch and ran the repaired FcmService through tsc; the only remaining diagnostics are unresolved module specifiers from --noResolve, so the brace structure now parses where it previously did not. The 9.22.0 to 12.4.0 service-worker CDN bump matches the firebase: ^12.4.0 dependency. Returning onMessage's unsubscribe from the new Observable(...) subscriber function is also correct RxJS teardown, so that sample no longer leaks its listener.

Two nits, neither blocking:

1. message$: Observable<unknown> loses the payload type a reader would want — Observable<MessagePayload> would be more useful as a starting point.
2. Inside async deleteToken(), the call to the imported deleteToken(this.msg) is correct but reads like recursion at a glance. An import alias or a one-line comment would save the double-take.

Also worth noting for merge order: this and #3734 both touch docs/app-check.md and docs/auth.md with adjacent hunks. They don't duplicate each other, but whichever lands second should be rebased and re-skimmed rather than merged on a stale green.