fix: recurse into HTML elements within ICU branches to extract interpolation placeholders

[Brooooooklyn]

When ICU case text contains interpolations inside HTML elements like <strong>{{ expr }}</strong>, extract_placeholders_from_nodes was silently dropping them because it only handled Text and Expansion nodes. This caused fewer i18nExp calls than Angular emits (e.g., 5 instead of 8 for undo-toast-items.component.ts with nested ICU plurals).

Add HtmlNode::Element recursion to match Angular's i18n_parser.ts visitElement behavior which recursively visits children.

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Note

Low Risk
Small, localized change to ICU placeholder extraction with a targeted regression test; main risk is subtle ordering/count differences in generated i18nExp arguments for templates with ICU + nested markup.

Overview
Fixes ICU placeholder extraction so i18n processing no longer drops interpolations nested inside HTML elements within ICU branches by recursing into HtmlNode::Element children in extract_placeholders_from_nodes.

Adds an integration test plus snapshot covering nested ICU plurals with markup like <strong>{{ expr }}</strong>, asserting the generated ɵɵi18nExp(...) chain includes the expected expressions (e.g., ctx.name, ctx.category).

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