Fix remote fonts not recognised when using a brand extension

[gordonwoodhull]

Summary

Fixes #13685

• Fix issue where remote font URLs in brand.yml are incorrectly treated as file paths when the brand file is in a subfolder (like an extension)
• Add isExternalPath helper to detect URLs (paths starting with a protocol like https:) and skip path joining for external font paths
• Add ensureCssRegexMatches test assertion for verifying CSS content in supporting files

Test plan

• Added smoke test that verifies:
  • CSS contains correct font URL (src:url("https://notofonts.github.io/...)
  • CSS does NOT contain broken path (_extensions/my-brand/https:)
• Test passes locally

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[gordonwoodhull]

It hallucinated ensureCssRegexMatches, which I've often wanted, so I told it to ask deepwiki how to add the assertion.

Implementation is messy and IIUC just looks for CSS files in output directory - probably should be refined before merging.

[cderv]

It hallucinated ensureCssRegexMatches, which I've often wanted, so I told it to ask deepwiki how to add the assertion.

This is interesting! I think we did use playwright for this kind of CSS verification until now. Mainly to be sure that the CSS applied in browser is the one expected. Or at least this is how I tried to use some CSS value check using playwright.

[cderv]

FWIW we have this

quarto-cli/src/core/url.ts

Lines 10 to 12 in ba576a8

	export function isHttpUrl(url: string) {
	return /^https?:/i.test(url);
	}

but yours is more generic. Also, if you need this one, you should probably add it in one of the core files as we already have several times

quarto-cli/src/core/brand/brand.ts

Lines 275 to 278 in ba576a8

	function isExternalPath(path: string) {
	return /^\w+:/.test(path);
	}

quarto-cli/src/project/types/website/website-navigation.ts

Lines 1543 to 1545 in ba576a8

	function isExternalPath(path: string) {
	return /^\w+:/.test(path);
	}

quarto-cli/src/project/types/website/website-utils.ts

Lines 123 to 125 in ba576a8

	function isExternalPath(path: string) {
	return /^\w+:/.test(path);
	}

Maybe it is time to have this helper in one place.