markdown: escape alt text in images

[srawlins]

See b/536194819

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[gemini-code-assist]

Code Review

This pull request ensures that image description text is properly escaped when assigned to the HTML 'alt' attribute, addressing potential XSS vulnerabilities. It introduces a new utility function, 'escapeAttributeCharactersValue', to escape quotes and angle brackets without double-escaping existing ampersands, and adds several test cases to verify this behavior. The review feedback recommends optimizing the utility function by utilizing Dart's 'HtmlEscape' with a custom 'HtmlEscapeMode' instead of chained 'replaceAll' calls, and warns of a potential issue where a null 'alt' attribute could be converted to the literal string 'null' during mapping.

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// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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markdown	❗ Show Issues These packages may be unused, or you may be using assets from these packages: * meta

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File	Coverage
pkgs/markdown/lib/src/inline_syntaxes/image_syntax.dart	💚 100 %
pkgs/markdown/lib/src/util.dart	💚 100 %

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[natebosch]

Can you add me on the buganizer?

[natebosch]

Why is it that we end up with somethign here which is partially escaped? Where would we have escaped & but not these others?

[srawlins]

Yeah this is awkward. By the time the alt text has been put into an element's textContent, it has already passed through EscapeHtmlSyntax, emitting escaped Text nodes. So if you have input like ![a & " b](source), then the text that comes out of node.textContent, in the edited lines above, has a & " b.

This escaping is done early because we parse the alt text into more inline nodes, which is allowed by the spec. The spec allows, for example ![foo *bar*](source), such that the description has inline formatting. In HTML the inline formatting is dropped, producing foo bar alt text. But the model is supposed to parse and keep track of that inner parsing (it also might be required for... correctly parsing erroneous text?).