Fix(kb): enforce tenant authorization on UpdateMetadataSetting (#15268) (#15270)

## Summary

Closes #15268.

The `UpdateMetadataSetting` handler at `internal/handler/kb.go:126`
retrieved the authenticated user via `GetUser(c)` but discarded the user
object (`_, errorCode, errorMessage := GetUser(c)`), then forwarded the
caller-supplied `kb_id` straight to the service layer with no ownership
check. Any authenticated user could mutate the `parser_config` /
metadata of any knowledge base in the system by guessing or harvesting a
`kb_id` — a classic IDOR (CWE-284, OWASP A01).

This is the only handler in `internal/handler/kb.go` missing the check;
every sibling (`ListTags`, `ListTagsFromKbs`, `RenameTag`,
`KnowledgeGraph`, `DeleteKnowledgeGraph`, `GetMeta`, `GetBasicInfo`)
already calls `h.kbService.Accessible(kbID, user.ID)`. The same
defensive check on the document preview endpoint was added in PR #14625
— this PR closes the matching gap on the KB metadata endpoint.

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Co-authored-by: Jin Hai <haijin.chn@gmail.com>