fix(emqx_schema): use non negative integer type for 'depth' SSL option

[SergeTupchiy]

Fixes EMQX-10276

Summary

🤖 Generated by Copilot at bfad459

Fixed a bug that prevented EMQ X broker from starting with SSL/TLS when the certificate chain depth was negative. Added a unit test for the server_ssl_opts_schema function to check the depth field validation.

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/{ce,ee}/(feat|perf|fix)-<PR-id>.en.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

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update

[yanzhiemq]

Bug Fixes

• Added validation to ensure that certificate depth (listener SSL option) is a non negative integer.

[yanzhiemq]

修复

• 增加了对证书层级（监听器 SSL 选项）须为非负整数的验证。