VisualStudioCredential throw CUE for all failures except task cancellation

[christothes]

fixes #50450

[Copilot]

Pull Request Overview

This PR standardizes the exception behavior for VisualStudioCredential by ensuring that all failures, except when cancellation is requested, throw CredentialUnavailableException.

• Test methods are refactored to remove the chained credential flag and now uniformly expect CredentialUnavailableException.
• The catch block in VisualStudioCredential is updated to remove legacy error string matching in favor of a cancellation check.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
sdk/identity/Azure.Identity/tests/VisualStudioCredentialTests.cs	Removed conditional handling based on a boolean flag and updated test expectations accordingly.
sdk/identity/Azure.Identity/src/Credentials/VisualStudioCredential.cs	Simplified exception handling by removing error string matching and adding cancellation logic.
sdk/identity/Azure.Identity/CHANGELOG.md	Updated changelog to describe the new exception throwing behavior.

Comments suppressed due to low confidence (1)

sdk/identity/Azure.Identity/src/Credentials/VisualStudioCredential.cs:122

• [nitpick] Local variable naming in C# typically uses camelCase. Consider renaming 'IsCancellationRequested' to 'isCancellationRequested' for consistency.

bool IsCancellationRequested = e is OperationCanceledException && cancellationToken.IsCancellationRequested;