fix bugs in AlignByDeviceOrderByLimitOffsetTest#orderByTimeTest

[chihyu0917]

Description

Motivation

AlignByDeviceOrderByLimitOffsetTest occasionally failed under NonDex because four methods orderByTimeTest1, orderByTimeTest2, orderByTimeTest3, and orderByTimeTest5 relied on fixed child-index positions in the plan tree. Planner variability (different data-region splits, rule ordering, parallelism, etc.) can reorder or insert nodes (ExchangeNode, extra TopKNode, etc.), so index-based assertions were nondeterministic.

Design & Implementation

• Searches for that node type among children (helper findRootTopK, firstTopKChild, etc.).
• Asserts 1–3 ExchangeNodes, matching the possible number of remote fragments.
• Accepts either SeriesAggregationScanNode or TopKNode, verifying that at least one fragment of each kind exists.
• Walks each sub-tree and verifies type presence (via containsNodeType helper).

Reproduce the error

mvn -pl iotdb-core/datanode edu.illinois:nondex-maven-plugin:2.1.7:nondex -Dtest=org.apache.iotdb.db.queryengine.plan.planner.distribution.AlignByDeviceOrderByLimitOffsetTest#orderByTimeTest1

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This PR has:

• [ ] been self-reviewed.
  • [ ] concurrent read
  • [ ] concurrent write
  • [ ] concurrent read and write
• [ ] added documentation for new or modified features or behaviors.
• [ ] added Javadocs for most classes and all non-trivial methods.
• [ ] added or updated version, license, or notice information
• [ ] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• [ ] added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage.
• [ ] added integration tests.
• [ ] been tested in a test IoTDB cluster.

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