fix(angular-query): ensure initial mutation pending state is emitted

[ThiloAschebrock]

Fixes #9020

The issue was that the effect that subscribes to the observer runs after the observer emits the first state change if the mutation is triggered in the constructor of a component or ngOnInit. This is fixed by immediately subscribing when the initial result is computed.

One could also consider combining resultFromSubscriberSignal and resultFromInitialOptionsSignal into a single linkedSignal, but this would require dropping support for Angular 18.