fix(a11y): restore icon visibility in Windows High Contrast Mode

[bmatar]

Description

In Windows High Contrast Mode (Edge / Chromium with forced-colors: active), several icon SVGs disappear because their paths use hardcoded fills (or no fill, defaulting to black). Chromium's spec-correct default forced-color-adjust: preserve-parent-color for SVG preserves these author colors against the HCM-substituted Canvas, so a black-fill icon on HCM-dark Canvas sits at 1.00:1 contrast.

Affected controls: operation expand chevrons, lock indicators (auth state), the modal close icon, the Authorize-button lock, JSON-schema accordion chevrons, the model expand toggle, copy-to-clipboard buttons, the dark-mode form checkmark.

In the released swagger-ui (no dark-mode plugin), these icons render as black-on-black in HCM dark — completely invisible. With the dark-mode plugin active, the plugin's CSS coincidentally sets non-default colors on the SVG element, so they render at low contrast (gray-on-black) rather than full disappearance. The fix addresses both.

Two layers: (1) inline icon SVG paths use fill="currentColor" so the inherited (HCM-substituted) color cascades to the path, with a @media (forced-colors: active) block in _buttons.scss pinning the paths to ButtonText for theme-independent contrast and resetting the unlocked-icon opacity dimming that would otherwise composite the system color back to ~40% alpha. (2) Data-URL background-image chevrons get @media blocks that re-render the same SVG shape as a mask-image filled by background-color: ButtonText/CanvasText.

Motivation and Context

Refs #7350 (the existing accessibility issue lists "Expand collapsed chevrons and Unlock controls present in the End Points tab section is not visible highcontrast black mode" as a HCM finding).

Detected by Tactual. WCAG 1.4.11 Non-text Contrast.

Screenshots

All captured under forced-colors: active. Built from the same swagger-ui commit on both ports, only this patch differing.

Operations list — page-light + HCM dark (no dark-mode plugin, what petstore.swagger.io ships):

BEFORE	AFTER

Operations list — page-dark + HCM dark (with dark-mode plugin):

BEFORE	AFTER

Models section — page-light + HCM dark:

BEFORE	AFTER

Operations list — page-light + HCM light:

BEFORE	AFTER

Out of scope

• <select> chevron in HCM (the <select> background-image chevron, both released _topbar.scss HTTPS dropdown variant and the dark-mode rule at _dark-mode.scss:156): <select> is a replaced element with no usable pseudo-element, so the mask-image approach used here can't apply without wrapping the <select> in a markup change. Followup PR.
• Native control color-scheme in dark mode: PR fix(dark-mode): indicate dark color-scheme for native browser controls #10844 adds color-scheme: dark to html.dark-mode for native scrollbars/inputs. Independent and non-conflicting; HCM substitutes system colors regardless of color-scheme.

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

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• If yes to above: I have updated the documentation accordingly.

Automated tests

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• If yes to above: I have added tests to cover my changes.
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• All new and existing tests passed.