[BUG] Token-limiter resets the Redis TTL on every increment, so the time-window never resets under sustained traffic

- severity: High
- files: `shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-token-limiter/src/main/java/org/apache/shenyu/plugin/ai/token/limiter/AiTokenLimiterPlugin.java:177-183`
- description: `recordTokensUsage` does `opsForValue().increment(cacheKey, tokens).flatMap(currentValue -> reactiveRedisTemplate.expire(cacheKey, Duration.ofSeconds(windowSeconds))).subscribe()`. Because `expire` is re-issued after every increment, as long as requests keep arriving within `windowSeconds` the key never expires and the counter is never reset. The intended "token budget N per window" becomes a monotonic lifetime cap.
- impact: Time-window token quotas do not reset on schedule; sustained traffic gets permanently throttled until a quiet period.
- suggested_fix: Set the TTL only on key creation (e.g. `INCR` then `EXPIRE NX`, or a small lua that sets expiry only when `EXISTS` is false).
- confidence: High
- related_existing: none — #6513/#6514/#6515 are different.

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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 shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-token-limiter/src/main/java/org/apache/shenyu/plugin/ai/token/limiter/AiTokenLimiterPlugin.java:177-183 and trace recordTokensUsage. Verify that the Redis TTL is not refreshed by each increment and that the token window resets under sustained traffic; done means the quota resets on schedule.

Written by the indexing model from the issue text.