SeaweedFS: Path traversal in the S3 and Iceberg REST gateways allows cross-bucket access

## Summary The S3 API gateway and the Iceberg REST catalog gateway construct their routers with `mux.NewRouter().SkipClean(true)`. With path cleaning disabled, a `..` segment inside the URL survives routing, so a request such as: ``` GET /bucket-A/../evil-bucket/key ``` is matched as `bucket=bucket-A`, `object=../evil-bucket/key`. The captured object key is then joined into a filer path with `util.JoinPath` (S3) / `path.Join` (Iceberg), which collapse the `..` server-side, so the actual read or write lands in `evil-bucket`. The captured path variables were never validated for traversal segments before reaching the handlers, so bucket isolation depended on downstream checks rather than on the path itself. ## Impact - **With authentication disabled** (`enableAuth=false`): direct cross-bucket read and write. An object key containing `..` resolves to and operates on a different bucket than the one named in the request path. - **With authentication enabled** (`enableAuth=true`): an authorization confused-deputy. IAM evaluates the policy against the mux `{bucket}` variable (`bucket-A`) in `iam.authRequestWithAuthType`, while the I/O is performed against the traversed target (`evil-bucket`). A principal authorized for one bucket can therefore reach objects in another bucket it has no grant for. This breaks tenant isolation. The same class of traversal applies to the Iceberg REST catalog's `{prefix}`, `{namespace}`, and `{table}` path variables. `%2e%2e`-encoded and `..\` (backslash)