refactor(tests): improve notification assertions in...

refactor(tests): improve notification assertions in HttpServletSseServerTransportProviderIntegrationTests

    Replace index-based assertions with content-based lookups using a notification map.
    This change makes the tests more resilient by removing the dependency on
    notification order, which is important for asynchronous messaging tests.

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>