fix(ai): project empty message trace timelines

[yyqdbngt]

Summary

MessageTraceToolHandler projects the trace timeline into the tool output, but assumed MessageService.getMessageTrace always returns a record with populated nodes / consumerStatus lists. Providers may report a missing trace as null (the service's own recordTraceQuery already guards for exactly that case) or as a record whose lists are absent, and the handler turned both shapes into an NPE 500 for AI/MCP/CLI callers.

The projection now treats a missing trace or missing lists as an empty timeline. The tool output schema only requires the arrays to be present, so empty arrays stay schema-valid.

Why

The same trace lookup is also served to the web UI, where a missing trace renders as an empty timeline. MCP/CLI callers of rmq.message.trace should get the same "no trace found" answer instead of an internal error.

Testing

Extended MessageTraceToolHandlerTest with two regression cases: null trace record, and record with null node/consumer-status lists.

mvn -q -Dtest=MessageTraceToolHandlerTest test
Tests run: 3, Failures: 0, Errors: 0, Skipped: 0

[RockteMQ-AI]

Summary

Fixes NPE in MessageTraceToolHandler when the trace record or its node/consumer-status lists are null. Now projects empty timelines instead of crashing with a 500. Good defensive coding with two test cases covering both null-trace and null-lists scenarios.

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