Add null validation httpsclientslim

[DarianBaker]

Add null validation to HttpClientSlim Uri parameters

• You've read the Contributor Guide and Code of Conduct.
• You've included unit or integration tests for your change, where applicable.
• You've included inline docs for your change, where applicable.
• There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.

Add null validation for Uri parameters in public API methods

Description

This PR adds ArgumentNullException.ThrowIfNull(requestUri) validation to public methods in HttpClientSlim that accept Uri parameters but don't currently validate them.

Methods updated:

• GetStringAsync(Uri requestUri, bool validateCertificate)
• PostAsync(Uri requestUri, HttpContent content, bool validateCertificate)
• GetStream(Uri requestUri, bool validateCertificate) (private helper)
• GetSocket(Uri requestUri)

Rationale to updating:
Without null validation, callers passing null would receive a NullReferenceException deep in the method implementation. This change provides feedback with ArgumentNullException instead which can improving diagnostics to make the API contract better explicit.

Impact:

• No breaking changes, only adds validation
• Better erroring for invalid input
• Should be consistent with the .NET API design guidelines

[dotnet-policy-service]

Thanks for your PR, @@DarianBaker. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.