[BUG] ProxySelectorServiceImpl.fetchData NPE on null discoveryHandler / discovery

- severity: Medium
- files: `shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/ProxySelectorServiceImpl.java:396-414`
- description: `discoveryHandlerMapper.selectById(discoveryHandlerId)` (398) is dereferenced at 399; `discoveryMapper.selectById(...)` (399) is dereferenced at 402/412. `proxySelectorDO` and `selectorDO` are null-checked, but handler and discovery are not. Exposed via PUT `/proxy-selector/fetch/{discoveryHandlerId}`.
- impact: Calling fetch with a stale handler id NPEs instead of returning a not-found result.
- suggested_fix: Guard `discoveryHandlerDO` and `discoveryDO`.
- 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)._

Open shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/ProxySelectorServiceImpl.java:396-414 and trace the PUT /proxy-selector/fetch/{discoveryHandlerId} path. Inspect the null handling around discoveryHandlerMapper.selectById and discoveryMapper.selectById. Done means stale handler or discovery IDs return a not-found result instead of causing an NPE.

Written by the indexing model from the issue text.