Fix bugs in AlignByDeviceOrderByLimitOffsetTest#orderByExpressionTest2 due to fixed child-index order

[chihyu0917]

Description

Motivation

AlignByDeviceOrderByLimitOffsetTest#orderByExpressionTest2 failed non-deterministically under NonDex
because the test assumed a fixed child-index order in the logical-plan tree.
When the planner inserts or reorders nodes (for example, adding an ExchangeNode before the first TopKNode), the original assertions either threw a ClassCastException or triggered an AssertionError.

Design and Implementation

• Locating the root TopKNode: Added findRootTopK(DistributedQueryPlan), which iterates over every `FragmentInstance and returns the first child that is a TopKNode
• Casting safety: Perform instanceof checks before casting, added assertNotNull for the root to fail fast if not found.
• Exchange node counting: Calculate rootTopK.getChildren().stream().filter(n instanceof ExchangeNode).count() and assert it equals 3 instead of checking fixed indices (1,2,3)

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#orderByExpressionTest2

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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