Align vertexai session service via ADK REST API with other impl - #1571

Problem:

The following route

		Route{
			Name:        "GetSession",
			Methods:     []string{http.MethodGet},
			Pattern:     "/apps/{app_name}/users/{user_id}/sessions/{session_id}",
			HandlerFunc: r.sessionController.GetSessionHandler,
		},

doesn't behave the same for different implementations.

For VertexAI Session Service the result of getting the session belonging to another user results in HTTP 500. For other implementations: HTTP 404.

Solution:
Aligning the vertexai implementation.