[BUG] AlertReceiverServiceImpl.updateReceiver uses full updateByPrimaryKey, nulling omitted columns #6711

• severity: Medium
• files: shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/AlertReceiverServiceImpl.java:78-82
• description: updateReceiver maps the DTO via AlertTransfer.INSTANCE.mapToAlertReceiverDO and calls alertReceiverMapper.updateByPrimaryKey(receiverDO) (full column update). Any DTO field not populated becomes NULL (e.g. namespaceId, levels, labels, matchAll, enable, config).
• impact: Partial PUT payloads silently wipe receiver configuration, which can disable alerting or change matching scope.
• suggested_fix: Switch to updateByPrimaryKeySelective(receiverDO), or require full DTO on update.
• confidence: Medium
• related_existing: none.

Identified during the 2026-08-02 deep re-scan; full list in docs/scan2-2026-08-02/06-medium-tiers.md.