fix(mcp): reuse configured response json mapper

[dengliming]

Summary

• inject the configured MCP JSON mapper into streamable HTTP results
• serialize MCP response objects with the transport mapper instead of a plain ObjectMapper
• route every streamable result construction through the configured mapper and add regression tests

Testing

• mvn -q -pl shenyu-plugin/shenyu-plugin-mcp-server -am -DskipTests=false -Dcheckstyle.skip=false -Dtest=MessageHandlingResultTest,ShenyuStreamableHttpServerTransportProviderTest -DfailIfNoTests=false test

Fixes #6749

[Aias00]

Approved as PMC (Aias00). Reviewed the diff — small, well-scoped fix with regression tests; mergeable.

[Voyagerroc-Lab]

Reviewed the MCP transport result construction and mapper injection. All response-result creation paths now route through the configured McpJsonMapper, preserving framework-specific serialization while string bodies still bypass serialization; the focused test verifies both behaviors. I found no blocking issue.