fix(core): effects wait for ngOnInit for their first run

[alxhub]

When an effect is created in a component constructor, it might read signals which are derived from component inputs. These signals may be unreliable or (in the case of the proposed input signals) may throw if accessed before the component is first change detected (which is what makes required inputs available).

Depending on the scenario involved, the effect may or may not run before this initialization takes place, which isn't a great developer experience. In particular, effects created during CD (e.g. via control flow) work fine, as do effects created in bootstrap thanks to the sync CD it performs. When an effect is created through dynamic component creation outside of CD though (such as on router navigations), it runs before the component is first CD'd, causing the issue.

In fact, in the signal components RFC we described how effects would wait until ngOnInit for their first execution for exactly this reason, but this behavior was never implemented as it was thought our effect scheduling design made it unnecessary. This is true of the regular execution of effects but the above scenario shows that creation of the effect is still vulnerable. Thus, this logic is needed.

This commit makes effects sensitive to their creation context, by injecting ChangeDetectorRef optionally. An effect created with an injector that's tied to a component will wait until that component is initialized before initially being scheduled. TestBed effect flushing is also adjusted to account for the additional interaction with change detection.

[alxhub]

This PR was merged into the repository by commit ee9605f.

[simeyla]

@alxhub Is there a tracking issue for this and other related developer preview features?

The specific concern I have is related to toObservable, given that it is now subject to this new timing (it uses effect).

Here's an oversimplified example where this may cause unexpected behavior.

 // settings from config service
const config = signal({ color: 'red' };  

ngOnInit() 
{
   // Angular 16 - subscribe runs immediately   
   // Angular 17 - subscribe runs after ngOnInit
   toObservable(this.config).subscribe((config) =>
   {
       this.service.initialize(config.color);
   });

   if (this.service.color() == 'red')
   {
      // this will never be reached in Angular 17
   }
}

Maybe toObservable should simply set the initial value immediately and not rely on the effect?

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