fix tests in ColumnInjectionPushDownTest

[kabo87777]

Fix Non-Deterministic Behavior in ColumnInjectionPushDownTest

Problem

Two tests were failing non-deterministically under NonDex with ~50% failure rate:

• testCannotPushDownTimeJoinAlignByTime
• testPartialPushDownTimeJoinAlignByDevice

Reproduce

cd iotdb-core/datanode
mvn edu.illinois:nondex-maven-plugin:2.1.1:nondex \
  -Dtest=ColumnInjectionPushDownTest#testCannotPushDownTimeJoinAlignByTime \
  -DnondexRuns=4

Failure example: java.lang.AssertionError: expected:ColumnInjectNode@79d58e5b but was:ColumnInjectNode@d7938b9b

Root Cause

1. HashMap in test utilities - Non-deterministic iteration order caused plan nodes to be constructed differently across runs
2. Order-sensitive comparison in FullOuterTimeJoinNode - Children were compared as ordered lists, but join order doesn't affect semantic meaning

Solution

1. Test utilities: Replaced HashMap with LinkedHashMap to preserve insertion order

   • OptimizationTestUtil.schemaMap
   • OptimizationTestUtil.getAggregationDescriptor()
   • All deviceToMeasurementIndexesMap in tests

2. Production code: Implemented order-independent equals() in FullOuterTimeJoinNode

   • Children are now compared as sets rather than ordered lists
   • Semantically correct since full outer time join order doesn't matter

Key Changed Classes

• OptimizationTestUtil - HashMap → LinkedHashMap
• ColumnInjectionPushDownTest - HashMap → LinkedHashMap
• TestPlanBuilder - HashMap → LinkedHashMap
• FullOuterTimeJoinNode - Order-independent equals()

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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