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

---

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.

---

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