Fix pipe idempotent IT setup synchronization

[Caideyipi]

Description

Fix the remaining setup race in IoTDBPipeIdempotentIT after #18316.

The pipe applies configuration events asynchronously. Waiting for the initial database snapshot does not guarantee that the statements in beforeSqlList (for example, CREATE USER) have reached the receiver. The test can therefore create the receiver-side idempotent conflict too early and fail with errors such as User newUser does not exist.

Queue a database marker after the setup statements and wait until it is visible on the receiver before creating the conflict. Queue a second marker after the conflict to verify that the pipe continues processing subsequent events.

Tests

• mvn spotless:check -pl integration-test -P with-integration-tests
• mvn '-Ddevelocity.off=true' -P 'with-integration-tests,MultiClusterIT2DualTableManualEnhanced' -pl integration-test -am '-DskipUTs' '-DintegrationTest.forkCount=1' '-Dit.test=IoTDBPipeIdempotentIT#testTableAlterUserIdempotent3' '-Dfailsafe.failIfNoSpecifiedTests=false' verify

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This PR has:

• been self-reviewed.
• added comments explaining the synchronization point.
• modified an existing integration test to cover the race.
• been tested in a local dual-cluster IoTDB environment.