Fix Azure Resource ID sanitization documentation and tests

[Copilot]

Multiple tests and comments incorrectly documented Azure Resource ID sanitization as using dash (-) when the implementation uses underscore (_).

Changes

Tests corrected:

• BaseClusterModel.test.ts: Two test cases now use .replace(/\//g, '_') instead of .replace(/\//g, '-')
• DiscoveryBranchDataProvider.test.ts: Fixed test to expect error throw (matches implementation) instead of warning

Comments corrected:

• VCoreBranchDataProvider.ts: Updated inline comment to "replace '/' with '_'"
• RUBranchDataProvider.ts: Updated inline comment to "replace '/' with '_'"

All changes align with the actual sanitizeAzureResourceIdForTreeId() implementation:

export function sanitizeAzureResourceIdForTreeId(azureResourceId: string): string {
    return azureResourceId.replace(/\//g, '_');  // underscore, not dash
}

These issues were previously marked as resolved but the incorrect documentation remained in the codebase.

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