The original implementation initializes auth session using direct struct
creation `&rtl.AuthSession` which does not have a source field
initialized, causing nil pointer error when the sdk is trying to access
this source field. This is fixed by using the`NewAuthSession()`
constructor which automatically initializes a source field.
Fix: https://github.com/googleapis/genai-toolbox/issues/2230
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Co-authored-by: Dr. Strangelove <drstrangelove@google.com>