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);

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[Aias00]

Switching the mapper to updateByPrimaryKeySelective preserves omitted columns in the database, but the cache update still replaces the whole cached rule with ScaleRuleDO.buildScaleRuleDO(scaleRuleDTO). For a partial update, any field left null in the DTO remains unchanged in the DB but becomes null in ScaleRuleCache, because addOrUpdateRuleToCache stores that partial object directly. Could we refresh the persisted row after the selective update, or merge the existing cached/DB rule before updating the cache, so the in-memory scale rule matches the database after partial updates?

[juicewcode]

Switching the mapper to updateByPrimaryKeySelective preserves omitted columns in the database, but the cache update still replaces the whole cached rule with ScaleRuleDO.buildScaleRuleDO(scaleRuleDTO). For a partial update, any field left null in the DTO remains unchanged in the DB but becomes null in ScaleRuleCache, because addOrUpdateRuleToCache stores that partial object directly. Could we refresh the persisted row after the selective update, or merge the existing cached/DB rule before updating the cache, so the in-memory scale rule matches the database after partial updates?

This change updates scale-rule handling in two ways:

• Use updateByPrimaryKeySelective so fields omitted from the request remain unchanged in the database.
• Reload the persisted ScaleRuleDO after the update and use the complete entity to refresh the cache, preventing omitted
  fields from becoming null in the cache.

The stale cache entry caused by changing metricName is addressed separately in PR #6975