fix: sliding dots have white background

[OEvgeny]

Fixes #

Changelog Entry

• Fixed Fluent/Copilot typing indicator animation background color, in PR #5770, by @OEvgeny

Description

This removes fluent/copilot sliding dots animation background so it always look neat independent of the page background.

Design

Produce asset which is frozen instead of using imperative SVG APIs.

Specific Changes

• Added new asset and asset name

• Used the asset when reduced motion preference is active

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[Copilot]

Pull request overview

Updates the Fluent/Copilot typing indicator “sliding dots” asset to remove the unwanted white background and adds a reduced-motion variant to avoid imperative SVG animation control.

Changes:

• Added a reduced-motion “sliding dots” asset entry and wiring to select it when prefers-reduced-motion is active.
• Simplified the SlidingDots component to render as an <img> instead of an <object> with SVG DOM control.
• Updated/expanded HTML-based typing indicator tests to support Fluent/Copilot dark theme scenarios and React 18 module loading.

Reviewed changes

Copilot reviewed 8 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
packages/test/test-assets/entry/@fluentui/tokens.js	Exports webDarkTheme for test pages that need Fluent dark theme tokens.
packages/fluent-theme/src/components/assets/SlidingDots.tsx	Switches to <img> and selects a reduced-motion asset by name.
packages/fluent-theme/src/components/assets/AssetName.ts	Adds a new asset name for reduced-motion sliding dots.
packages/fluent-theme/src/components/assets/AssetComposer.tsx	Generates and registers a reduced-motion SVG blob URL alongside the existing sliding dots asset.
tests/html2/typing/typingIndicator.scroll.html	Migrates test page setup to React 18/module import maps and adds Fluent theme + color-scheme emulation support.
tests/html2/typing/typingIndicator.scroll.fluent.dark.html	Adds a redirecting test fixture for Fluent dark theme.
tests/html2/typing/typingIndicator.scroll.copilot.dark.html	Adds a redirecting test fixture for Copilot (Fluent variant) dark theme.
CHANGELOG.md	Adds changelog entry for the typing indicator background color fix.

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[compulim]

I love the idea of reduced-motion in asset name. I was puzzled abit about the replaceAll but it makes sense.