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Add diagnostics for idle prefetch triggers

Since adding support for @defer (on idle(500ms)), some idle + prefetch timing combinations are not detected correctly.

@defer (on idle(500ms); prefetch on idle(200ms) ) {
    <heavy-comp />
}

Currently, this throws an error that is actually incorrect

NG8021: Prefetch 'idle' matches the main trigger and provides no benefit. Remove the prefetch modifier.

Adds warning for prefetch without main defer trigger

Relative to #52746 and according examples in documentation

For example

@defer (prefetch when someFlag) {
  <heavy-comp />
}

This configuration may suggest that the prefetch will only run when someFlag becomes true.
However, since the main trigger still defaults to on idle, the deferred content can be fetched earlier during the browser’s idle period, effectively bypassing the intended condition.

Closes #52746