[BUG] — `AiResponseTransformerPlugin`: `new ObjectMapper()` per response (two sites)

• severity: High
• files: shenyu-plugin-ai/shenyu-plugin-ai-response-transformer/.../AiResponseTransformerPlugin.java:220 (new ObjectMapper().readTree(body) inside extractJsonBodyFromAiResponse, per response) and :337 (ObjectMapper objectMapper = new ObjectMapper() inside writeWith's flatMap, per response)
• description: writeWith runs on every decorated response body when the ai-response-transformer plugin is active; new ObjectMapper() builds a fresh TypeFactory/SerializerProvider each call. Sibling AiTokenLimiterPlugin.java:196 correctly hoists a static MAPPER.
• impact: Per-response ObjectMapper construction on every AI-transformed response; CPU + young-gen pressure under load.
• suggested_fix: Hoist private static final ObjectMapper MAPPER = new ObjectMapper(); (reuse sibling pattern).
• confidence: High
• related_existing: PERF-22 covers McpServerPlugin.java:710 + MessageHandlingResult.java:93; these are different files/classes.

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Identified during the 2026-08-02 deep re-scan; full list in docs/scan2-2026-08-02/00-consolidated-critical-high.md.