Ensuring ValidateCachedTokenAsync works for other apis

[trwalke]

This pull request introduces a unified approach for validating cached access tokens before use, improving consistency and reliability across authentication flows. The main change is the addition of a new helper method, ValidateCachedAccessTokenAsync, which centralizes the logic for validating cached tokens using the authentication operation (when supported). This method is now used in multiple request types, and comprehensive unit tests have been added to ensure correct behavior.

Token validation improvements:

• Added the ValidateCachedAccessTokenAsync method in RequestBase.cs to centralize and streamline validation of cached access tokens using IAuthenticationOperation2. The method returns the original cache item if validation passes, or null if validation fails.
• Updated ClientCredentialRequest, OnBehalfOfRequest, and CacheSilentStrategy to use the new helper method for validating cached access tokens, replacing previous inline validation logic. [1] [2] [3]

Testing enhancements:

• Added unit tests in AuthenticationOperationTests.cs to verify that cached tokens are properly validated and either used or ignored based on the result of ValidateCachedTokenAsync. Tests cover both On-Behalf-Of and silent authentication flows, for both validation success and failure scenarios.

Code cleanup:

• Removed unnecessary using Microsoft.Identity.Client.AuthScheme; directive from ClientCredentialRequest.cs as part of refactoring.Ensuring ValidateCachedTokenAsync works for other apis