fix: schema for /gateways/:name/clients was missing top-level structure

[sstrigler]

Fixes EMQX-7820

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• [ ] Added tests for the changes
• [ ] Changed lines covered in coverage report
• [ ] Change log has been added to changes/<version>/(feat|fix)-<PR-id>.en.md and .zh.md files
• For internal contributor: there is a jira ticket to track this change
• [ ] If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• [ ] Schema changes are backward compatible

[coveralls]

Pull Request Test Coverage Report for Build 4233772772

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 50 unchanged lines in 18 files lost coverage.
• Overall coverage decreased (-0.08%) to 80.761%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_dashboard/src/emqx_dashboard_monitor.erl	1	69.44%
apps/emqx/src/bpapi/emqx_bpapi.erl	1	95.83%
apps/emqx/src/emqx_broker.erl	1	84.83%
apps/emqx/src/emqx_flapping.erl	1	85.11%
apps/emqx/src/emqx_quic_stream.erl	1	75.0%
apps/emqx/src/emqx_session.erl	1	87.6%
apps/emqx_gateway/src/emqx_gateway_conf.erl	2	89.12%
apps/emqx_gateway/src/emqx_gateway_insta_sup.erl	2	66.01%
apps/emqx_gateway/src/mqttsn/emqx_sn_frame.erl	2	64.04%
apps/emqx/src/emqx_config_handler.erl	2	96.21%

Totals
Change from base Build 4233018396:	-0.08%
Covered Lines:	23949
Relevant Lines:	29654

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💛 - Coveralls