Fix calc() computation in trimmed / NativeAOT applications

[sebastienros]

Problem

Computing any calc() that combines two metric values throws in NativeAOT (and trimmed) apps:

System.MissingMethodException: No parameterless constructor defined for type 'AngleSharp.Css.Values.CssLengthValue'.
   at AngleSharp.Css.Values.CssCalcAddExpression.AngleSharp.Css.Dom.ICssValue.Compute(ICssComputeContext)
   at AngleSharp.Css.Dom.CssProperty.Compute(ICssComputeContext)

Repro (NativeAOT, net10.0): <style>p { width: calc(100px + 20px) }</style> + GetComputedStyle returns 120px on JIT but throws under AOT.

The four calc expressions build their result with Activator.CreateInstance(x.GetType(), result). The trimmer cannot follow that, so it removes the CssXyzValue(Double) constructors that nothing else calls. Publishing an app with these packages surfaced this as 4 IL2072 trim warnings — the only trim warnings coming out of AngleSharp.Css (AngleSharp, AngleSharp.Xml and AngleSharp.XPath produce none).

Change

• Moved the reflection call into a single internal helper ICssMetricValue.WithValue(Double) that roots the public constructors of the eight built-in metric values via [DynamicDependency], plus a documented [UnconditionalSuppressMessage] for IL2072. ICssMetricValue is public, so the dynamic path is kept for external implementations — those simply have to preserve their own single-Double constructor (the justification text says so).
• Did not enable IsAotCompatible in this bug-fix PR. That property should be considered in a later release because it also opts the assembly into trimming; enabling it now could cause unexpected trimming in applications referencing AngleSharp.Css.
• Added tests: computed calc() addition/subtraction for lengths and times, and a WithValue round-trip for all eight metric types.

Verification

• dotnet test src/AngleSharp.Css.Tests/AngleSharp.Css.Tests.csproj → 2084 passed.
• dotnet build src/AngleSharp.Css.sln -c Release → 0 warnings.
• NativeAOT publish (osx-arm64, net10.0) of a sample app against this branch → 0 IL warnings, and calc(100px + 20px) / calc(100px - 20px) now compute 120px / 80px just like on JIT.

Note: calc(2 * 8px) and calc(30px / 2) still fail with InvalidOperationException: Unsupported unit cannot be converted. — that is a pre-existing issue that reproduces identically on JIT and is out of scope here.

[CLAassistant]

All committers have signed the CLA.

[FlorianRappl]

Great catch - thanks!