Reject "root" culture name in ICU globalization mode

[tarekgh]

Summary

Fixes#132429.

CultureInfo.GetCultureInfo("root") was accepted in ICU globalization mode but produced an incomplete culture. ICU normalizes the CLDR root locale to an empty name, so the resulting CultureData had Name == "" without being the Invariant singleton. This caused two problems:

1. Reading NumberFormat on it threw NullReferenceException from NumberFormatInfo.InitializeInvariantAndNegativeSignFlags because the negative sign was never populated.
2. Resolving "root" before the invariant culture poisoned the "" cache slot, so later lookups of the invariant culture returned the broken instance.

Fix

The empty-string and "und" names are already short-circuited to the invariant culture in CultureData.GetCultureData, so an empty name reaching InitIcuCultureDataCore can only come from a name such as "root" that is not a valid culture. The change rejects that case, so GetCultureInfo("root") now throws CultureNotFoundException, matching the existing NLS behavior (Windows does not recognize root).

This is a single length check on the culture creation cold path (cache miss only), so there is no impact on the hot path or on any valid culture.

Behavior by globalization mode

Mode	Before	After
ICU (default)	Incomplete culture, NRE on NumberFormat, cache poisoning	CultureNotFoundException
NLS	CultureNotFoundException	Unchanged
Invariant (predefined-only)	CultureNotFoundException	Unchanged
Invariant, predefined-only disabled	Fabricated culture named root from invariant data	Unchanged

Tests

Added to System.Globalization.Tests.GetCultureInfoTests (gated on IsNotInvariantGlobalization):

• GetCultureInfo_RootCultureName_Throws for root, ROOT, Root across all overloads and the CultureInfo constructor.
• GetCultureInfo_RootCultureName_DoesNotPoisonInvariantCache using RemoteExecutor to verify that resolving root first leaves the invariant culture intact and that und still maps to invariant.

Verified locally on Windows x64 (ICU): the full GetCultureInfoTests class passes (65/65) with the fix, and the new tests fail without it.

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🟢 Approval recommended

The change is a narrowly-scoped correctness fix on a cold path with targeted regression tests validating both the exception behavior and the cache-poisoning scenario.

Pull request overview

This PR fixes an ICU-mode globalization edge case where CultureInfo.GetCultureInfo("root") could succeed but produce a broken culture (with Name == ""), leading to internal NullReferenceExceptions and poisoning the CultureInfo name cache entry for the invariant culture.

Changes:

• Reject ICU-normalized empty locale names during ICU culture initialization to ensure "root" fails culture creation rather than producing an incomplete culture.
• Add regression coverage ensuring "root" throws CultureNotFoundException across key entry points and does not poison the invariant culture cache (including verifying "und" still maps to invariant).
• Gate the tests to only run when a non-invariant globalization backend is in use and RemoteExecutor is available.

File summaries

File	Description
src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.Icu.cs	Adds a guard to fail ICU culture initialization if ICU returns an empty locale name (preventing "root" from creating a broken culture and avoiding cache poisoning).
src/libraries/System.Runtime/tests/System.Globalization.Tests/CultureInfo/GetCultureInfo.cs	Adds regression tests verifying "root" throws and does not poison the invariant culture cache (and that "und" still maps to invariant).

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite