fix(compiler): check @defer blocks in the control flow content projection diagnostic - #71398

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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

The content of a @defer block always goes to the default ng-content slot, because the block doesn't copy the tag name and attributes of its root node (#58110, tracked in #64504). NG8011 doesn't check @defer blocks, so this happens without any warning:

<app-card>
  @defer {
    <h2 card-title>Title</h2>
  }
</app-card>

<h2 card-title> is projected into the default slot instead of [card-title], or not rendered if the component has no default slot.

Issue Number: Fixes #71397

What is the new behavior?

TcbControlFlowContentProjectionOp also goes through the @defer blocks of the element, including their @placeholder, @loading and @error blocks, and reports each root node that matches a specific slot. Unlike the other blocks, a single root node is reported too, since it isn't projected either.

The suggestions in the existing message (keep a single root node) don't apply here, so the out-of-band recorder gets a separate deferredBlockPreventingContentProjection method. It uses the same error code and the same controlFlowPreventingContentProjection option, and points to ngProjectAs:

Node matches the "[card-title]" slot of the "Card" component, but will not be projected into the specific slot because it is inside a @defer block. The content of @defer blocks, including their @placeholder, @loading and @error blocks, is always projected into the default slot, or not rendered if the component doesn't have one. To project the node in the right slot, wrap the @defer block in an <ng-container/> with an "ngProjectAs" attribute that matches the "[card-title]" selector.

I added four tests: the main block, the secondary blocks, content that only matches the catch-all slot, and the ngProjectAs workaround. The two that expect a diagnostic fail without the change. Like the other specs of this diagnostic, they're listed in ngp_known_failures.ts.

Does this PR introduce a breaking change?

• Yes
• No

Other information

The generated code doesn't change. The new warning only shows up for a @defer block placed directly inside a component with ng-content selectors, when one of its root nodes matches a specific selector. That node is already in the wrong slot today, so each warning points to a real bug, but it can be new on code that compiles cleanly now. If that's a problem for the internal rollout, I can retarget the PR.

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