[BUG] AiProxy stream retry replays the whole upstream, duplicating SSE content

- severity: High
- files: `shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-proxy/src/main/java/org/apache/shenyu/plugin/ai/proxy/enhanced/service/AiProxyExecutorService.java:88-115`; consumption at `AiProxyPlugin.java:171-182`
- description: `executeStream` wraps `doChatStream` in `Retry.max(1)` then `.onErrorResume(NonTransientAiException.class, handleFallbackStream)`. When the upstream stream errors AFTER emitting some chunks, Reactor `retryWhen` re-subscribes, issuing a brand-new upstream call emitting from the beginning. Those first-attempt chunks were already written to the client SSE response, so the client receives `[partial attempt-1][full attempt-2]`.
- impact: Corrupted/duplicated AI completions for streaming calls that hit a mid-stream error; non-idempotent streaming calls are replayed.
- suggested_fix: For streaming, do not blanket-retry mid-stream; retry only before any emission (buffer-first) or fail and let the client reconnect.
- confidence: High
- related_existing: none — #6575/#6583 concern the generic retry plugin, not the AI-proxy `executeStream` Flux replay.

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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`](docs/scan2-2026-08-02/00-consolidated-critical-high.md)._

Read executeStream in shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-proxy/src/main/java/org/apache/shenyu/plugin/ai/proxy/enhanced/service/AiProxyExecutorService.java:88-115 and the stream consumption in AiProxyPlugin.java:171-182. Trace the retry behavior after partial emission, then exercise a stream that errors mid-response. Done means the client no longer receives duplicated SSE content from replayed upstream output.

Written by the indexing model from the issue text.