i18n: Localise fileName and downloadButtonText of the File block

[bph]

Summary

• Adds core/file to get_text_replacement_patterns_for_html() and the dispatch switch in escape_text_content_of_blocks() so the File block's two rich-text-sourced attributes — fileName and downloadButtonText — are wrapped with esc_html_e() on theme save.
• The block.json declares "selector": "a:not([download])" for fileName and "selector": "a[download]" for downloadButtonText. Two regexes mirror that: a negative-lookahead pattern for the file-name link, and a \sdownload[\s>] pattern for the download button.
• Refs i18n: Additional blocks not made translation-ready on theme save #825 — second of the two pieces called out there. JSON-attribute additions shipped in i18n: Localize label attribute of additional blocks #826; this is the rich-text path.

Why a separate path

fileName and downloadButtonText are "source": "rich-text", so the values live as inner-HTML text inside the two <a> tags rather than in the block-comment JSON. escape_block_attribute_strings() (the JSON path) can't see them, so they need handling alongside figcaption, alt, and <summary>.

Test plan

• npm run lint:php clean
• npm run test:unit:php — 5 new tests pass (file, file without download button, file with custom button text, file with formatted file name, idempotency); 99/99 overall
• npm run build succeeds
• Studio smoke through parse_blocks → escape_text_content_of_blocks → serialize_blocks → escape_block_attribute_strings confirms both attributes wrapped with the active theme's text domain
• Reviewer to spot-check on a theme with a real File block via the Save Theme flow

Notes

• Behaviour change: themes saved with "Localize Text" enabled will now have File blocks rewritten — they were untouched before. Desired, but worth flagging.
• The regex assumes the current core HTML shape (boolean download attribute on the second anchor). If a future Gutenberg release restructures the block (e.g. swaps <a download> for <button>), the download-button regex stops matching. Same fragility as every other entry in get_text_replacement_patterns_for_html() — accepted for the band-aid approach tracked in Consider the ideal approach for text localization #776.