feat: rename log.file.to to log.file.path

[zhongwencool]

Fixes

Summary

🤖 Generated by Copilot at 4dfe3c5

Renamed to key to path in log_file_handler config and updated schema, tests, and config builder accordingly. This improves clarity and consistency of the config format and eases migration.

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]

Enhancements

• Renamed log.file.to to log.file.path.

[yanzhiemq]

增强

• 将 log.file.to 更名为 log.file.path。