fix: add range validation for max_clientid_len

[terry-xiaoyu]

Fixes

Summary

🤖 Generated by Copilot at 38d92c9

Add a new validator for MQTT client identifier lengths and use it in emqx.schema. This ensures that the configuration option mqtt.max_clientid_len is within the valid range of 23 to 65535.

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