[To dev/1.3] [Pipe] Drop builtin sinks synchronously (#18646)

[Caideyipi]

Description

Backport #18646 (e967831ca2a7922983e6dd1b27c2d41031574b9d) to dev/1.3.

Built-in sinks now wait for in-flight connector operations before discarding queued events or closing, so shutdown completes deterministically. External sinks keep the existing non-blocking discard and bounded close behavior.

dev/1.3 Adaptation

• Preserve the legacy connectorIndex and outputPipeConnector APIs.
• Build BUILTIN_SINKS only from plugins available on dev/1.3; the IoT Consensus V2 aliases and display-task-ID/receiver-session APIs do not exist on this branch.

Validation

• mvn spotless:apply -pl iotdb-core/node-commons,iotdb-core/datanode: passed.
• Targeted Java compilation of the three changed DataNode production classes and PipeSinkSubtaskTest: passed.
• PipeSinkSubtaskTest: passed, 10 tests.
• The normal DataNode Maven test lifecycle was also attempted, but local generated Thrift/ANTLR dependency artifacts are out of sync with the current dev/1.3 sources and compilation stops on unrelated missing types such as TShowRepairDataPartitionTableProgressResp and RepairDataPartitionTableContext before tests run.
• git diff origin/dev/1.3...HEAD --check: passed.

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This PR has:

• been self-reviewed.
  • concurrent read and write
• added or updated unit tests for the changed code paths.

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Key changed/added classes

• PipeSinkSubtask
• PipeSinkSubtaskManager
• SubscriptionSinkSubtask
• BuiltinPipePlugin
• PipeSinkSubtaskTest