fix(auth): check hasattr before accessing request.session Fixes #16035

[Bharath420]

The code was checking hasattr(request, 'session') in the outer if condition but then accessing request.session in the inner if without the hasattr check, causing AttributeError on Request objects.

This fix ensures we check hasattr(request, 'session') before accessing request.session to prevent AttributeError.

Problem:

• Users on Python 3.14 experienced AttributeError: 'Request' object has no attribute 'session'
• Occurred when using google-genai client with Vertex AI

Solution:

• Modified _prepare_request_for_mds() to check attribute existence before access
• Changed outer if condition from if use_mtls and hasattr(request, "session"): to if use_mtls:
• Added proper hasattr check in inner condition

Testing:

• Verified fix prevents AttributeError
• No breaking changes to existing functionality
• Maintains backward compatibility