Repository: angular
Issue: Improvement: Identifying defer and blocks for testing and perhaps other stuff
Labels: feature,area: core,core: defer

### Which @angular/* package(s) are relevant/related to the feature request?

core

### Description

Currently, on the [tests of the new Angular site](https://angular.dev/guide/defer) we finally see some unit tests for Defer loading, with mainly this syntax:

```
// Retrieve the list of all defer block fixtures and get the first block.
const deferBlockFixture = componentFixture.getDeferBlocks()[0];
// Render loading state and verify rendered output.
await deferBlockFixture.render(DeferBlockState.Loading);
expect(componentFixture.nativeElement.innerHTML).toContain('Loading');
```

For this component:
```
@defer {
  <large-component />
} @loading 
 Loading...
}
```

Now I'm guessing it just scans the HTML for blocks and picks the first one. But what would happen if I add a new defer block before my code, it will trigger the new defer block and not the old one. Hence, my existing tests will fail and if you have a few defer blocks in a single component, it would perhaps accept a few tests that are meant for a different block.

### Proposed solution

A way to identify the block so it is easier to test and perhaps also reference. I'm not entirely sure what all the use cases can be for this but I'm sure you guys can figure it out quickly. Chaining defer blocks and triggering other defer blocks seems like an inventive use case that could make Angular standout by just using the template to trigger other components and service calls.

So instead of the previous block we write:
```
@defer (as myDeferBlock) {
  <large-component />
} @loading {
  Loading...
}
```

And in the tests I can now use:
```
// Retrieve the list of all defer block fixtures and get the first block.
const deferBlockFixture = componentFixture.getDeferBlock('myDeferBlock');
// Render loading state and verify rendered output.
await deferBlockFixture.render(DeferBlockState.Loading);
expect(componentFixture.nativeElement.innerHTML).toContain('Loading');
```

This is a similar syntax to how async variables get referencing:
```
@if (users$ | async; as users) {
  {{ users.length }}
}
```
And you can also chain it with triggers: 

Perhaps with triggers you can add it at the back: `@defer (on hover; as myDeferBlock) {`

I also see an added benefit of using the identity to trigger another defer or to have another component with no direct relation to the defer, to know when a service is done loading an item in its template. An API could be used in the other component to trigger on defer loading with something like:

```
constructor (private deferDoneLoading: DeferDoneLoading) {}

ngOnInit() {
  this.deferDoneLoading.onLoaded('myDeferBlock').subscribe((value) => this.myFunctionToRunOnChildLoaded(someValue));
}
```

Or another defer `@defer (on myDeferBlock) {` so you can chain requests and data without having to add a single subscription, just pure templates.

### Alternatives considered

The alternative would be to not have it at all and just rely on an array that may or may not be filled. Especially when mixing defer blocks into for-loops it could be problematic to know how many there will be and in what order they will exist. And having to add broker services that trigger other events or adding a lot of subscriptions for the sake of triggering other subscriptions is tedious and takes more time to test when they could be talking to each other directly.

The syntax is not set in stone, I don't know the inner logic well enough to know what is and isn't possible, but if one could identify or store a reference to a defer block some way, it would make a lot of interesting things possible.