Fix bug blocking DPG and update CoreResponseClassifier constructor to take ReadOnlySpan<ushort>

[annelo-msft]

Two changes:

• We are currently disregarding the ResponseClassifier passed to CreateMessage if RequestContext is null. We need to use it to set the RC and then modify the RC iff RequestContext has modifications.
• Making a small API update to CoreResponseClassifier per API feedback: update CoreResponseClassifier constructor to take ReadOnlySpan<ushort> to improve perf.

Note: Creating as draft to raise any early concerns and will add tests before taking out of draft

[azure-sdk]

API changes have been detected in Azure.Core. You can review API changes here

API changes

-         public CoreResponseClassifier(ReadOnlySpan<int> nonErrors);
+         public CoreResponseClassifier(ReadOnlySpan<ushort> successCodes);

[annelo-msft]

This PR is currently blocked by this: #27350 (comment)