New POP test - Sni Assertion Flow Uses JwtPop And Succeeds

[gladjohn]

This pull request adds a new integration test to validate the client assertion flow with JWT PoP (Proof of Possession) binding in the ClientCredentialsMtlsPopTests class. The new test ensures that MSAL correctly uses the jwt-pop client assertion type when a binding certificate is supplied and PoP is enabled. Additional minor improvements include new constant definitions and using extra namespaces.

New test for JWT PoP client assertion flow

• Added Sni_AssertionFlow_Uses_JwtPop_And_Succeeds_TestAsync() to verify that MSAL uses the jwt-pop client assertion type and includes the correct body parameters when acquiring tokens with mTLS PoP and a client assertion. The test also checks the correct invocation of the assertion provider and validates the returned access token and scopes.

Test setup improvements

• Defined a new constant TokenExchangeUrl for use in token acquisition flows.
• Added comments clarifying the POP test requirements regarding the allow-listed SNI app and tenant.

Namespace and dependency updates

• Added System.Linq, System.Threading, and Microsoft.Identity.Client.Extensibility namespaces to support new test logic and features.