fix(pulsar): use a binary duration as default health_check_interval

[thalesmg]

Fixes https://emqx.atlassian.net/browse/EMQX-9885

The frontend needs the default value to match the duration (binary) type to display correctly.

Summary

🤖 Generated by Copilot at 5960cc5

Changed the default value and format of health_check_interval option for Pulsar bridge plugin. Updated override_default function to support binary values.

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