fix(storage): give the compat fromTask an explicit return type

fromTask declared no return type, so the published typings import
whatever path the typings bundler picks for the inferred firebase
type. In 20.x that is 'firebase/compat', which firebase's exports map
does not list. Apps using moduleResolution "bundler", the setting the
@angular/build migration moves apps to, fail with TS2307 in
@angular/fire/compat/storage. 21.0.0-rc.1 happens to emit
'firebase/compat/app' and compiles, but ng-packagr 22.2's new typings
bundler cannot parse the inferred type and fails the library build.

Declaring Observable<UploadTaskSnapshot> removes the inferred import.
The type is unchanged: UploadTaskSnapshot is the compat alias for
firebase.storage.UploadTaskSnapshot.

Also drop a comment explaining a firebase import that #3421 removed
in 2023 as unused. The import only existed to steer these typings.

Fixes #3677

Author: armando-navarro

src/compat/storage/observable/fromTask.ts

@@ -2,12 +2,9 @@ import { Observable } from 'rxjs';
 import { debounceTime } from 'rxjs/operators';
 import { UploadTask, UploadTaskSnapshot } from '../interfaces';
 
-// need to import, else the types become import('firebase/compat/app').default.storage.UploadTask
-// and it no longer works w/Firebase v7
-
 // Things aren't working great, I'm having to put in a lot of work-arounds for what
 // appear to be Firebase JS SDK bugs https://github.com/firebase/firebase-js-sdk/issues/4158
-export function fromTask(task: UploadTask) {
+export function fromTask(task: UploadTask): Observable<UploadTaskSnapshot> {
   return new Observable<UploadTaskSnapshot>(subscriber => {
     const progress = (snap: UploadTaskSnapshot) => subscriber.next(snap);
     const error = e => subscriber.error(e);