[ColorProvider] Extending brand colors override functionality to macOS

[DanielMorM]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

This PR extends the FluentUI theming system to support customizable brand colors through GlobalTokens, enabling apps to override the default Communication Blue color palette (brand10-brand160) with their own brand colors.

New Protocol - BrandColorProviding.swift:

• Introduces BrandColorProviding protocol with 16 brand color properties (brand10 through brand160)
• Cross-platform support for iOS, visionOS, and macOS using PlatformColor typealias

TokenSet.swift:

• Added methods to dynamically update/remove overrides: setOverride(:for:), setOverrides(:), removeOverride(for:), removeAllOverrides()
• Changed valueOverrides from immutable to mutable to support runtime updates

FluentTheme.swift:

• Added public methods for color override management: setColorOverrides(:), setColorOverride(:for:), removeColorOverride(for:), removeAllColorOverrides()
• Added automatic didChangeTheme notification posting when modifying the shared theme

GlobalTokens.swift:

• Added static brandColorProvider: BrandColorProviding? property
• Updated brandSwiftUIColor(_:) to return colors from the provider when set, falling back to Communication Blue defaults

ColorProviding.swift (iOS):

• ColorProviding now inherits from BrandColorProviding
• setColorProvider(_:) now updates the theme in-place instead of replacing it
• setSharedThemeColorProvider(_:) now also sets GlobalTokens.brandColorProvider

ColorProviding.swift (macOS):

• ColorProviding now inherits from BrandColorProviding
• Added new brand background/foreground color properties
• Added setSharedThemeColorProvider(_:) extension method (parity with iOS)

Colors.swift (macOS):

• Setting Colors.colorProvider now automatically calls FluentTheme.setSharedThemeColorProvider() for backward compatibility

Demo Apps:

• Added brand color assets for green and purple themes (iOS) and demo brand colors (macOS)
• Updated DemoColorGreenTheme, DemoColorPurpleTheme, and ObjectiveCDemoColorProviding to implement BrandColorProviding
• Updated macOS TestColorViewController to display brand colors with toggle support

Binary change

[Pending]

Verification

• Built and ran iOS demo app with green and purple theme switching
• Built and ran macOS demo app with color provider toggle
• Verified brand colors update correctly via GlobalTokens.brandSwiftUIColor()
• Verified FluentTheme.shared color overrides update in-place
• Verified backward compatibility: setting Colors.colorProvider on macOS updates the shared theme
• Tested Objective-C interoperability via ObjectiveCDemoColorProviding

Visual Verification

Scenario	Screenshot
Before
After - NOT using color provider
After - using color provider

Pull request checklist

This PR has considered:

• Light and Dark appearances
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow

[mischreiber]

Can you walk me through why we have these secondary definitions for brand colors in the iOS app, compared to what was already present?

[mischreiber]

We should not make global tokens conditional; there are legit scenarios where a branded app may still want to access the default comm blue colors. All overrides should happen at the theme level for this reason.