[BUG] TagServiceImpl.update is not transactional yet performs recursive multi-row updates before updating the parent

## Description
`update` is not annotated `@Transactional`. It calls `updateSubTags(tagDTO)` which loads all tags, rebuilds the `ext` chain, and recursively calls `tagMapper.updateByPrimaryKey(tagDO)` for every descendant, *then* updates the parent itself. If the parent update (or any descendant update) fails, the descendants that already succeeded are left with `ext` values computed from the new parent while the parent row still holds the old values — a torn tree.

## Location
```
shenyu-admin/.../service/impl/TagServiceImpl.java:86-92 (recursion at :156-193)
```

## Impact
Sub-tree `ext` metadata becomes inconsistent with the parent; some children updated, parent/others not, with no rollback.

## Suggested fix
Annotate `update` with `@Transactional(rollbackFor = Exception.class)`.

## Related existing issue(s)
#6520 is a different NPE on missing `parentTagId`; not a dup.

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Start in shenyu-admin/.../service/impl/TagServiceImpl.java at update (lines 86-92), then trace the recursive updateSubTags path at lines 156-193. Check how the multi-row updates behave when a descendant or the parent update fails; done means the whole update rolls back instead of leaving inconsistent ext values.

Written by the indexing model from the issue text.