Remove duplicated duration validation logic in `DefaultRedisCacheWriter`.

[youngsuk-kim]

Current Code

The same validation pattern appears twice:

• Line 215: !sleepTime.isZero() && !sleepTime.isNegative()
• Line 566: !this.sleepTime.isZero() && !this.sleepTime.isNegative()

Proposed Solution

Extract this logic into a helper method isPositiveDuration(Duration) to improve code maintainability and readability.