[BUG] ProxySelectorServiceImpl.update NPE on null discoveryRel / handler / discovery

- severity: Medium
- files: `shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/ProxySelectorServiceImpl.java:347-394`
- description: `discoveryRelMapper.selectByProxySelectorId(proxySelectorDO.getId())` (353) is dereferenced at 354; `discoveryHandlerMapper.selectById(discoveryHandlerId)` (355) is mutated at 358; `discoveryMapper.selectById(...)` (364) is mutated at 366; `proxySelectorAddDTO.getDiscovery()` is dereferenced at 365-368 with no null check. Reached via `createOrUpdate` (PUT `/{id}`).
- impact: Updating a proxy selector whose binding was deleted, or omitting `discovery` in the body, throws NPE rather than a 4xx.
- suggested_fix: Null-check each lookup and return a clear error.
- confidence: Medium
- related_existing: #6517 is about omitted `discoveryUpstreams`; this is the omitted/missing `discovery` and missing rel/handler/discovery derefs, a separate path.

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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/ProxySelectorServiceImpl.java at lines 347-394 and trace the createOrUpdate PUT /{id} path. Exercise updates with an omitted discovery object and with deleted or missing discovery bindings. Done means these cases return a clear 4xx error instead of throwing an NPE.

Written by the indexing model from the issue text.