[BUG] Response transformer only intercepts writeWith; streaming/chunked responses bypass transformation

- severity: Medium
- files: `shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-response-transformer/src/main/java/org/apache/shenyu/plugin/ai/transformer/response/AiResponseTransformerPlugin.java:282-380`
- description: `AiResponseTransformerDecorator` overrides only `writeWith`. `ServerHttpResponseDecorator.writeAndFlushWith` delegates to the underlying response, so any upstream that writes via `writeAndFlushWith` (chunked/SSE) flows through with no transformation. By contrast `AiTokenLimiterPlugin.AiStatisticServerHttpResponse` overrides BOTH.
- impact: Streaming AI responses are silently left un-transformed despite the plugin being configured.
- suggested_fix: Override `writeAndFlushWith` and route inner publishers through the same transform.
- confidence: Medium-High
- related_existing: none

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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)._

Open AiResponseTransformerPlugin.java:282-380 and inspect how AiResponseTransformerDecorator handles writeWith. Compare it with AiTokenLimiterPlugin.AiStatisticServerHttpResponse, which overrides both response-writing methods. Done means chunked or SSE writes through writeAndFlushWith receive the same transformation as writeWith.

Written by the indexing model from the issue text.