Fix viewBox SVG size when root tag is not at start - #16149

Fixes #16128.

Problem

getImageDetails() sets the size of a viewBox-only SVG from its viewBox. This code had three problems:

• It ran only if the SVG text started with <svg . An SVG with an XML declaration, a comment, or leading whitespace skipped it. Chrome then loaded the SVG at its default size (150 × 150), so the filtered copy showed cropped or empty in a small element.
• includes('width=') also matched stroke-width= on the root element, so such an SVG also skipped the code.
• The rebuilt SVG used svgText.slice(5), which assumes that the root tag is at index 0.

Changes

• Find the first <svg followed by whitespace or >, and read the opening tag from that position.
• Require whitespace before the width, height, and viewbox attribute names.
• Insert the width and height attributes after <svg at the found position, and keep the text before it.

A comment before the root element that contains <svg text would match first. This is rare, and a full prolog parser needs much more code.

Tests

Six new tests in tests/inject/dynamic/image-analysis.tests.ts:

• XML declaration, leading comment, leading whitespace, and stroke-width on the root element. These four fail without the change (useViewBox is false, and the size is 150 × 150).
• A plain viewBox-only SVG and an SVG with a real size. These two pass with and without the change.

npm run test:inject passed in headless Chrome 156 (146 of 146), and npm run lint and npm test passed.