[BUG] MessageHandlingResult.getResponseBodyAsJson serializes MCP types with plain ObjectMapper (wrong serializer)

- severity: Medium
- files: `shenyu-plugin/shenyu-plugin-mcp-server/src/main/java/org/apache/shenyu/plugin/mcp/server/transport/MessageHandlingResult.java:87-98`
- description: `getResponseBodyAsJson()` does `new ObjectMapper().writeValueAsString(responseBody)`. When `responseBody` is a `McpSchema.JSONRPCMessage`, serialization uses a vanilla ObjectMapper without the MCP Jackson module that the configured `JacksonMcpJsonMapper` would apply. MCP sealed-record/polymorphic types may serialize incorrectly.
- impact: Streamable HTTP responses may be malformed when the body is an MCP schema object.
- suggested_fix: Reuse the transport's configured `McpJsonMapper`/`ObjectMapper` instead of `new ObjectMapper()`.
- confidence: Medium
- related_existing: none — PERF-22 is perf only.

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._

Start in shenyu-plugin/shenyu-plugin-mcp-server/src/main/java/org/apache/shenyu/plugin/mcp/server/transport/MessageHandlingResult.java:87-98 and inspect getResponseBodyAsJson(), then trace the transport's configured McpJsonMapper/ObjectMapper. Done means Streamable HTTP responses serialize MCP schema objects with the configured MCP-aware mapper rather than a vanilla ObjectMapper.

Written by the indexing model from the issue text.