perf(config): enforcing atom key path in hotcode path

[qzhuyan]

Fixes EMQX-9618

Summary

🤖 Generated by Copilot at a8cfebe

Refactored and optimized runtime configuration access in emqx_config.erl. Used a consistent type of runtime_config_key_path() and removed redundant code.

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

[coveralls]

Pull Request Test Coverage Report for Build 5001345957

• 21 of 26 (80.77%) changed or added relevant lines in 7 files are covered.
• 155 unchanged lines in 23 files lost coverage.
• Overall coverage decreased (-0.02%) to 81.851%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/emqx/src/emqx_config.erl	6	11	54.55%

Files with Coverage Reduction	New Missed Lines	%
apps/emqx_bridge_dynamo/src/emqx_bridge_dynamo_connector_client.erl	1	65.31%
apps/emqx_conf/src/emqx_conf_schema.erl	1	92.86%
apps/emqx_resource/src/emqx_resource_buffer_worker.erl	1	94.32%
apps/emqx_rule_engine/src/emqx_rule_sqlparser.erl	1	91.67%
apps/emqx/src/emqx_logger.erl	1	80.49%
apps/emqx/src/emqx_message.erl	1	94.83%
apps/emqx/src/emqx_stats.erl	1	91.67%
apps/emqx/src/emqx_sys_mon.erl	1	90.63%
apps/emqx_authz/src/emqx_authz_api_schema.erl	2	96.72%
apps/emqx_gateway_mqttsn/src/emqx_mqttsn_frame.erl	2	63.48%

Totals
Change from base Build 4981883058:	-0.02%
Covered Lines:	27127
Relevant Lines:	33142

---

💛 - Coveralls

[yanzhiemq]

Enhancements

• Optimized memory usage when accessing the configuration during runtime.

[yanzhiemq]

增强

• 优化了在运行时访问配置的内存使用。