fix: Use selective updates for scale rules(#6709)

[juicewcode]

Fixes #6709
ScaleRuleServiceImpl#update previously called updateByPrimaryKey, which performs a full-column update.
When a partial update request omits optional fields such as minimum, maximum, sort, or status, those fields may
be written as NULL, unintentionally removing existing scale rule configuration.

Changes

Replace:
scaleRuleMapper.updateByPrimaryKey(after);
with:
scaleRuleMapper.updateByPrimaryKeySelective(after);

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• You submit test cases (unit or integration tests) that back your changes.
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