Fix Creating Undermined Culture

[tarekgh]

Fixes #115913

The culture name und (undetermined) is a special case recognized by ICU. It typically gets normalized to an empty string, which maps to the invariant culture. The fix here ensures that und is handled consistently with the invariant culture and prevents overwriting the cached invariant culture entry when creating a culture for und.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-globalization
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull Request Overview

This PR ensures that the ICU-normalized “und” (undetermined) culture maps to the invariant culture and prevents duplicate caching.

• Added a UndeterminedCultureTest to verify name normalization and caching behavior
• Updated GetCultureInfo to only insert new entries if absent
• Modified NormalizeCultureName to treat empty ICU results (like “und”) as invariant

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Runtime/tests/System.Globalization.Tests/CultureInfo/CultureInfoCtor.cs	Added a test for “und” normalization and caching of invariant culture
src/libraries/System.Private.CoreLib/src/System/Globalization/CultureInfo.cs	Changed cache insertion to TryAdd so existing entries aren’t overwritten
src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.cs	Treated empty-window-name ICU results as invariant culture

Comments suppressed due to low confidence (2)

src/libraries/System.Runtime/tests/System.Globalization.Tests/CultureInfo/CultureInfoCtor.cs:425

• Consider adding an assertion that undetermined1 references the same instance as the invariant culture (e.g., Assert.Same(invariant1, undetermined1)) to verify caching behavior for "und".

CultureInfo undetermined1 = CultureInfo.GetCultureInfo("und");

src/libraries/System.Private.CoreLib/src/System/Globalization/CultureInfo.cs:1053

• You can simplify cache insertion by using nameTable.GetOrAdd(name, result) instead of manual lock and TryAdd, reducing code complexity.

lock (nameTable)

[ericstj]

Just a couple small comments. Otherwise seems reasonable.

[tarekgh]

/ba-g build timeout and unrelated