Add comprehensive test cases for cuddAddApply.c targeting 90% coverage

[Copilot]

Expand test coverage for src/cuddAddApply.c from 18.9% toward the 90% target by adding tests for all ADD apply operators.

Changes

• Binary operators: Tests for Cudd_addPlus, Cudd_addTimes, Cudd_addThreshold, Cudd_addSetNZ, Cudd_addDivide, Cudd_addMinus, Cudd_addMinimum, Cudd_addMaximum, Cudd_addOneZeroMaximum, Cudd_addDiff, Cudd_addAgreement
• Logical operators: Tests for Cudd_addOr, Cudd_addNand, Cudd_addNor, Cudd_addXor, Cudd_addXnor
• Monadic operators: Tests for Cudd_addMonadicApply and Cudd_addLog
• Recursive paths: Tests exercising cuddAddApplyRecur and cuddAddMonadicApplyRecur with non-constant ADDs

Each operator test covers terminal cases including:

• Identity elements (zero, one)
• Special values (plus/minus infinity, background)
• Equal operands
• Operand swap paths
• Constant vs non-constant ADDs

TEST_CASE("Cudd_addThreshold - F >= G", "[cuddAddApply]") {
    DdManager *dd = Cudd_Init(0, 0, CUDD_UNIQUE_SLOTS, CUDD_CACHE_SLOTS, 0);
    DdNode *c5 = Cudd_addConst(dd, 5.0);
    DdNode *c3 = Cudd_addConst(dd, 3.0);
    Cudd_Ref(c5); Cudd_Ref(c3);
    DdNode *result = Cudd_addApply(dd, Cudd_addThreshold, c5, c3);
    REQUIRE(result == c5);
    // cleanup...
}

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Original prompt

Complete test cases for src/cuddAddApply.c and target 90% coverage for the source file.

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