Implement IFormattable on HslColor and HsvColor

[NathanDrake2406]

What does the pull request do?

Adds IFormattable support to HslColor and HsvColor, following the same convention established in #20919 for Color: uppercase includes alpha, lowercase excludes it.

Type	Specifier	Output	Example
HslColor	L	hsl(h, s%, l%, a)	hsl(240, 80%, 20%, 0.5)
HslColor	l	hsl(h, s%, l%)	hsl(240, 80%, 20%)
HsvColor	V	hsv(h, s%, v%, a)	hsv(240, 80%, 20%, 0.5)
HsvColor	v	hsv(h, s%, v%)	hsv(240, 80%, 20%)

Default ToString() behavior is unchanged for both types.

What is the current behavior?

HslColor and HsvColor have a single ToString() that outputs a fixed full-precision format (hsla(h, s, l, a) / hsva(h, s, v, a)). There is no way to control alpha inclusion or get CSS-style percentage output.

What is the updated/expected behavior with this PR?

var hsl = new HslColor(0.5, 240, 0.8, 0.2);
hsl.ToString("L", null) // "hsl(240, 80%, 20%, 0.5)" — with alpha
hsl.ToString("l", null) // "hsl(240, 80%, 20%)"       — without alpha
hsl.ToString()          // unchanged default

How was the solution implemented (if it's not obvious)?

Same pattern as Color (#20919): IFormattable interface guarded by #if !BUILDTASK, format specifier selects output, IFormatProvider accepted but ignored (culture-invariant). Hue is rounded to integer degrees, saturation/lightness/value to integer percentages — standard CSS representation.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None. IFormattable is additive and default ToString() is unchanged.

Fixed issues

Fixes #20927

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0063583-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[robloo]

Note: The existing code will output hsva() if there is an alpha channel.

We should double check the CSS spec and see if that was a mistake originally and if that needs to change one way or the other.

[robloo]

HSV was a natural extension of HSL which is part of the CSS standard: https://www.w3schools.com/css/css_colors_hsl.asp

HSV is how the ColorPicker code in upsteam UWP/WinUI worked and also aligned with the community toolkit's HsvColor so was added first. Then I added HSL to give us the full CSS color models support. Since HSL follows CSS, HSV follows the same formatting rules which does include hsv vs hsva.

[robloo]

Here is a copy/paste bug in the legacy code. This should be hsla

Since it is going to have to change and break backwards compatibility we can definitely decide if there should be an a or not based on what CSS does.

No one noticed this before anyway so it isn't used for anything I would think.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0063641-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[NathanDrake2406]

Superseded by #20919 which now implements the unified format specifier design across all three color types per @robloo's review feedback.