[FLINK-23555] Support common sub-expression elimination in code generation

[featzhang]

Implemented CSE optimization for Flink SQL code generation. Key changes:

New files:

• CseUtils.scala - Utility to scan RexNode trees and identify duplicate sub-expressions
• CseITCase.scala - Integration tests for CSE (correctness, call count, null handling, nested)
• CseTestFunctions.java - Test UDFs with AtomicInteger call counters
• CseUtilsTest.scala - Unit tests for the CSE analyzer

Modified files:

• ExprCodeGenerator.scala - Added cseEnabled, cseExprCache, cseCandidates fields and CSE logic in generateExpression(). Also changed operand visiting in visitCall to go through generateExpression (not accept(this)) so nested sub-expressions are also cached.
• CalcCodeGenerator.scala - Added CSE analysis before projection code generation using CseUtils.findDuplicateSubExpressions()

Approach: RexNode digest string is used as expression identity key. Only RexCall nodes appearing more than once are cached. First encounter generates code + stores in local variable. Subsequent encounters return NO_CODE reference to cached variable.

Test results (all passing):

• CseJavaITCase: 5/5 ✅ (testCseCorrectness, testCseCallCount, testNoCseCandidates, testCseWithNullValues, testCseWithNestedCommonSubExpressions)
• CseUtilsTest: 6/6 ✅

[flinkbot]

CI report:

• 938f7d9 Azure: FAILURE

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