fix(emqx_management): Ignore results from the nodes that are down

[ieQu1]

Fixes EMQX-9531

Summary

🤖 Generated by Copilot at 07ecfd5

Improve error handling of get_stats/1 in emqx_mgmt_api_stats.erl. Filter out nodes that return errors. Add emqx:running_nodes() API to avoid calling mria from EMQX code directly.

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

[sstrigler]

If I’m not confusing things, there was a related, recent incident with improperly formatted error messages due to a mix of string and binaries (my assumption), maybe you could fix this as well?

[coveralls]

Pull Request Test Coverage Report for Build 4730265706

• 39 of 39 (100.0%) changed or added relevant lines in 9 files are covered.
• 56 unchanged lines in 21 files lost coverage.
• Overall coverage decreased (-0.07%) to 81.197%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_authn/src/enhanced_authn/emqx_enhanced_authn_scram_mnesia.erl	1	86.57%
apps/emqx_bridge/src/emqx_bridge.erl	1	91.27%
apps/emqx_dashboard/src/emqx_dashboard_monitor.erl	1	65.28%
apps/emqx_modules/src/emqx_delayed.erl	1	94.78%
apps/emqx_rule_engine/src/emqx_rule_engine.erl	1	86.23%
apps/emqx/src/emqx_broker.erl	1	87.64%
apps/emqx/src/emqx_schema.erl	1	88.89%
apps/emqx/src/emqx_session.erl	1	87.65%
apps/emqx/src/emqx_stats.erl	1	91.67%
apps/emqx/src/proto/emqx_proto_v2.erl	1	0%

Totals
Change from base Build 4728296228:	-0.07%
Covered Lines:	25594
Relevant Lines:	31521

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💛 - Coveralls

[yanzhiemq]

@ieQu1 It seems EMQX-9531 has nothing to do with this PR