refactor: consolidate MCP integration tests into shared abstract base class

- Extract common test logic from WebMvcSseIntegrationTests and WebMvcStreamableIntegrationTests into AbstractMcpClientServerIntegrationTests
- Add json-unit-assertj dependency to mcp-test module for JSON assertion support
- Make McpServer.SyncSpecification abstract to enforce proper inheritance
- Remove duplicate test implementations (~2000+ lines of code deduplicated)
- Maintain parameterized testing across different client transport types (httpclient, webflux)

This refactoring eliminates significant code duplication between Spring WebMVC integration test classes while maintaining full test coverage and functionality.

Signed-off-by: Christian Tzolov <[email protected]>

Author: tzolov