test: fix flaky test EmbeddedDartReportApiTest

[FrankChen021]

Summary

• Wait for completed Dart query reports before asserting completion metadata.
• Apply the same synchronization to the single-broker completed-report test and the cross-broker test.

Root cause

The master CI failure was reported as expected: not <null> at EmbeddedDartReportApiTest.java:348, which is the assertion that DartQueryInfo.getDurationMs() is non-null. The report response itself was present.

The SQL result can be returned before ControllerHolder.runAsync finishes deregistration and DartControllerRegistry publishes the retained completed report. During that window, the report API can return the running-query snapshot, whose durationMs is explicitly null. Cross-broker forwarding can observe the same intermediate state.

Evidence:

• Failed master job
• Same test passed on the immediate parent run
• Report metadata is documented as null until known
• Controller deregistration publishes the completed report after query execution

Fix

Poll the report API until each requested broker returns a report with non-null durationMs, using one shared timeout for the multi-broker check. This synchronizes the test with the API's completed-report state without adding a fixed sleep.

Validation

mvn -ntp test -pl embedded-tests -am \
  -Dtest="org.apache.druid.testing.embedded.msq.EmbeddedDartReportApiTest" \
  -Dsurefire.failIfNoSpecifiedTests=false \
  -Pskip-static-checks -Dweb.console.skip=true -T1C

Result: 7 tests passed, 0 failures, 0 errors.