Add comprehensive test coverage for cuddAddInv.c (88.9% line coverage)

[Copilot]

Adds test cases for Cudd_addScalarInverse and cuddAddScalarInverseRecur in src/cuddAddInv.c, achieving 88.9% line coverage and 100% function coverage.

Tests Added

• Constant ADD operations: Inverse of various constants (2.0 → 0.5, 0.25 → 4.0, negatives)
• Invalid inputs: Non-constant epsilon returns NULL, values below epsilon threshold
• Multi-level ADDs: Single/multiple variable structures, 3-level deep ADDs
• Cache behavior: Verifies repeated calls return cached result
• Branch coverage: t == e optimization path, partial recursion failures
• Mathematical correctness: Validates inverse × original = 1

Example Test

TEST_CASE("Cudd_addScalarInverse - Constant ADD", "[cuddAddInv]") {
    DdManager *manager = Cudd_Init(0, 0, CUDD_UNIQUE_SLOTS, CUDD_CACHE_SLOTS, 0);
    
    DdNode *two = Cudd_addConst(manager, 2.0);
    Cudd_Ref(two);
    DdNode *epsilon = Cudd_addConst(manager, 1e-10);
    Cudd_Ref(epsilon);
    
    DdNode *result = Cudd_addScalarInverse(manager, two, epsilon);
    REQUIRE(result != nullptr);
    REQUIRE_THAT(Cudd_V(result), Catch::Matchers::WithinAbs(0.5, 1e-10));
    // ...
}

Uncovered Lines (4/36)

• Line 118: Timeout handler callback (requires CUDD_TIMEOUT_EXPIRED state)
• Lines 173-175: Memory exhaustion handling (requires cuddUniqueInter failure)

These are defensive error paths requiring runtime conditions not reliably reproducible in unit tests.

Original prompt

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

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