[Pipe] Stabilize IoTDBPipeAutoSplitIT for auto-dropped history pipe - #18621

Description

Fix the flaky IoTDBPipeAutoSplitIT assertion caused by a finite history-only pipe completing and being auto-dropped before SHOW PIPE.

• Require the persistent non-full-sync pipes a2b1 and a2b3 to remain.
• Allow a2b2 to be present or already auto-dropped, while rejecting unexpected or auto-split pipe names.
• Use DROP PIPE IF EXISTS so cleanup succeeds in either lifecycle state.

Verification

• mvn spotless:apply -pl integration-test -P with-integration-tests
• git diff --check
• Target integration test could not complete locally because the configured Maven mirror returned HTTP 504 and the corresponding snapshot dependencies were unavailable offline.

This PR has:

• been self-reviewed.
• added comments explaining the lifecycle race handled by the assertion.
• modified an existing integration test to cover the valid auto-drop state.

Key changed/added classes (or packages if there are too many classes) in this PR

• IoTDBPipeAutoSplitIT