Feat/rework mutators

[0xalpharush]

replaces #445

[dguido]

@0xalpharush - this mutator rework looks valuable! The PR has conflicts with recent master changes. Could you rebase when you have a chance? We'd like to review and merge this.

[dguido]

PR Review and Fixes

I've reviewed this PR and pushed fixes for several issues I found.

Merge Conflict Resolution

Resolved conflict in fuzzing/sequence_generator.go - kept the feature branch's AppendAndMutate naming while using the corrected "tail" spelling from master (was "tao;" typo in the feature branch).

Bug Fixes

1. Non-deterministic random number generation (Critical)

swapRandList, spliceAtRandom, and interleaveAtRandom were using global rand.Intn instead of the passed provider.Intn:

// Before (non-deterministic)
i, j := rand.Intn(len(xs)), rand.Intn(len(xs))

// After (deterministic)
i, j := provider.Intn(len(xs)), provider.Intn(len(xs))

This defeated the purpose of seeded deterministic fuzzing - test results would not be reproducible.

2. Potential panics on empty slices

spliceAtRandom and interleaveAtRandom would panic with rand.Intn(0) when passed empty slices. Added bounds checks:

func spliceAtRandom[T any](provider *rand.Rand, xs1, xs2 []*T) []*T {
    if len(xs1) == 0 {
        return xs2
    }
    if len(xs2) == 0 {
        return xs1
    }
    // ... rest of function
}

3. Missing mutation strategies

SpliceAndMutate and InterleaveAndMutate config fields were defined in CallSequenceGeneratorConfig but never wired up to the mutation strategy chooser in NewCallSequenceGenerator. Added them:

randomutils.NewWeightedRandomChoice(
    CallSequenceGeneratorMutationStrategy{
        CallSequenceGeneratorFunc: spliceCorpus,
        Mutate:                    true,
    },
    new(big.Int).SetUint64(config.SpliceAndMutate),
),
randomutils.NewWeightedRandomChoice(
    CallSequenceGeneratorMutationStrategy{
        CallSequenceGeneratorFunc: interleaveCorpus,
        Mutate:                    true,
    },
    new(big.Int).SetUint64(config.InterleaveAndMutate),
),

Code Quality Improvements

4. Test file cleanup

• Removed debug fmt.Println statements
• Removed commented-out code
• Fixed import path (github.com/crytic/medusa-geth/common instead of github.com/ethereum/go-ethereum/common)
• Renamed test from TestSplice to TestSequenceGenerationStrategies for clarity
• Added edge case tests for empty slice handling (TestSpliceAtRandomEmptySlices, TestInterleaveAtRandomEmptySlices)

Verification

• All tests pass ✅
• Build succeeds ✅
• go vet passes ✅