fix(core): skip let declarations during hydration cleanup - #71990

PR Checklist

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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?

@let values live directly in the LView. After hydration, cleanupLView walks those slots and treats any array with an object at index 1 as an LView, so an array stored in a @let gets i18nNodes/dehydratedIcuData written onto its 7th item. If that item is frozen, cleanup throws instead.

Issue Number: #71929

What is the new behavior?

cleanupLView skips @let slots, the same way annotateLView already does during serialization.

Does this PR introduce a breaking change?

• Yes
• No

Other information