refactor: extract common integration test logic into abstract base classes (#473)

refactor: extract common integration test logic into abstract base classes

- Move duplicated test methods from WebFlux and WebMvc integration test classes to abstract base classes
- WebFluxSseIntegrationTests, WebFluxStreamableIntegrationTests now extend AbstractMcpClientServerIntegrationTests
- WebFluxStatelessIntegrationTests, WebMvcStatelessIntegrationTests now extend AbstractStatelessIntegrationTests  
- Each concrete test class now only implements transport-specific setup methods (prepareClients, prepareAsyncServerBuilder, prepareSyncServerBuilder)
- Eliminates ~1300+ lines of duplicated test code across multiple files
- Improves maintainability by centralizing test logic in reusable base classes
- Updates WebMvcSseServerTransportProvider to use builder pattern
- Adds new HttpServletSseIntegrationTests extending AbstractMcpClientServerIntegrationTests
  - Removes HttpServletSseServerTransportProviderIntegrationTests
- Standardizes timeout configurations and client setup across all integration tests

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