Unify KC/KD normalization behavior on Wasm

[ilonatommy]

Fixes #117116.

• Call CheckNormalizationForm before the ASCII fast path so Browser/WASI throw PlatformNotSupportedException for FormKC/FormKD regardless of input.
• Add wasm-specific test for checking if PNSE was thrown for both, ASCII and non-ASCII with KC and KD.

Why didn't we catch it in tests?
Because we do not test KC/KD at all, considering them not supported (IsNotUsingLimitedCultures -> Browser with its ICU):

runtime/src/libraries/System.Runtime/tests/System.Globalization.Extensions.Tests/Normalization/StringNormalizationTests.cs

Line 60 in 0c185bc

if (PlatformDetection.IsNotUsingLimitedCultures || PlatformDetection.IsHybridGlobalizationOnApplePlatform)

[Copilot]

Pull Request Overview

This PR fixes a bug where Browser/WASI platforms inconsistently threw PlatformNotSupportedException for FormKC/FormKD normalization forms. The issue occurred when the input string was ASCII-only, as the exception was only thrown after the ASCII fast path check. The fix ensures that the exception is thrown consistently regardless of input by checking for unsupported forms earlier in the validation process.

Key changes:

• Moved compatibility form validation before the ASCII fast path to ensure consistent exception behavior
• Consolidated duplicate Browser/WASI guard logic into a reusable ThrowIfCompatibilityFormUnsupported method
• Added additional validation checks (Invariant and UseNls modes) to the guard conditions

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Private.CoreLib/src/System/Globalization/Normalization.cs	Added new ThrowIfCompatibilityFormUnsupported method and called it from CheckNormalizationForm to validate compatibility forms before ASCII fast path
src/libraries/System.Private.CoreLib/src/System/Globalization/Normalization.Icu.cs	Replaced inline Browser/WASI check with call to the new consolidated ThrowIfCompatibilityFormUnsupported method

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[tarekgh]

Left one final comment. LGTM otherwise.

[ilonatommy]

/ba-g windows-arm64 Release NativeAOT, windows-arm64 Release NativeAOT_Libraries and coreclr Pri0 Runtime Tests Run windows arm64 checked timeouts are not connected with the change.