fix(mcp): serialize message responses safely

[dengliming]

Summary

• serialize SSE message responses with Jackson instead of string interpolation
• emit UTF-8 JSON bytes without relying on the platform charset
• reuse the mapper for JSON error responses and cover escaping/non-ASCII content

Testing

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

Fixes #6748

[Aias00]

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