fix(storage): give the compat fromTask an explicit return type (#3796)

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);