feat: add WebFlux transport support to WebMVC integration tests

- Enhance McpStreamableServerSession error handling with proper JSON-RPC error responses
- Add mcp-spring-webflux test dependency to webmvc module
- Refactor WebMvcStreamableIntegrationTests to use parameterized tests
- Support testing with both HttpClient and WebFlux transports
- Add WebClientStreamableHttpTransport integration
- Improve error handling in McpStreamableServerSession
- Add proper timeout configurations for test clients
- Clean up test structure and imports organization

This enables comprehensive testing of WebMVC functionality with multiple
transport mechanisms, ensuring compatibility across different client types.

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