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

- 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.

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._

Start in shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/AlertReceiverServiceImpl.java:78-82 and inspect updateReceiver, the DTO mapping, and mapper call. Verify how a partial PUT is handled, then change the update behavior as proposed so omitted fields are preserved. Done means partial updates no longer null namespaceId, levels, labels, matchAll, enable, or config.

Written by the indexing model from the issue text.