feat(testing): added contract fuzzing and property-based testing

[Codex723]

D-11: Add Contract Fuzzing and Property-Based Testing

Summary

Integrates fuzzing tools and property-based testing frameworks to automatically discover edge cases and vulnerabilities in Soroban contracts. This PR adds 4 contract-specific fuzzing harnesses, 18 property-based tests across 6 test groups, seed corpora, CI pipeline integration, mutation testing coverage, and comprehensive documentation.

Changes

New Files

File	Description
fuzz/fuzz_targets/fuzz_wasm_validation.rs	WASM binary validation fuzzing harness — tests magic header, minimum size, and panic-freedom
fuzz/fuzz_targets/fuzz_contract_invocation.rs	Mock contract client invocation fuzzing harness — tests call counting, return/error determinism, and reset
fuzz/fuzz_targets/fuzz_contract_spec_parse.rs	Contract test spec JSON parsing fuzzing harness — tests malformed JSON, truncated docs, and hostile Unicode
fuzz/fuzz_targets/fuzz_test_generator.rs	Test case generation from source fuzzing harness — tests malformed Rust, empty files, and arbitrary fragments
tests/contract_property_tests.rs	18 property-based tests across 6 groups: WASM validation, WASM hash computation, mock contract invocation, mock storage, mock addresses, and mock environment
fuzz/corpus/fuzz_wasm_validation/valid_wasm.bin	Seed corpus for WASM validation fuzzing
fuzz/corpus/fuzz_contract_spec_parse/valid_spec.json	Seed corpus for contract spec parsing (valid spec)
fuzz/corpus/fuzz_contract_spec_parse/empty_spec.json	Seed corpus for contract spec parsing (empty spec)
fuzz/corpus/fuzz_contract_spec_parse/malformed_spec.json	Seed corpus for contract spec parsing (malformed JSON)
fuzz/corpus/fuzz_test_generator/valid_source.rs	Seed corpus for test generation (valid Rust source)
fuzz/corpus/fuzz_test_generator/empty_source.rs	Seed corpus for test generation (empty source)

Modified Files

File	Changes
fuzz/Cargo.toml	Added 4 new contract fuzz target entries + tempfile dependency
.github/workflows/fuzzing.yml	Added contract fuzz targets to fuzz-smoke matrix, added contract property tests to CI, fixed YAML indentation
FUZZING_GUIDE.md	Added contract fuzzing section with harness descriptions, invariants, run commands, and security focus areas
.cargo-mutants.toml	Added contract testing source files to mutation testing scope

Acceptance Criteria

• Property-based tests for contracts — 18 property tests across 6 groups in tests/contract_property_tests.rs
• Fuzzing harnesses work — 4 contract-specific harnesses in fuzz/fuzz_targets/
• Mutation testing integration — .cargo-mutants.toml updated to include contract source files
• Coverage reporting — CI pipeline includes cargo-llvm-cov coverage generation with Codecov upload
• CI pipeline integration — .github/workflows/fuzzing.yml updated with contract fuzz targets and property tests
• Documentation and examples — FUZZING_GUIDE.md updated with contract fuzzing section, examples, and security focus areas

How to Test

# Run contract property tests
cargo test --test contract_property_tests

# Run with more iterations
PROPTEST_CASES=5000 cargo test --test contract_property_tests

# Run contract fuzz targets (requires nightly + cargo-fuzz)
cargo fuzz run fuzz_wasm_validation --fuzz-dir fuzz -- -max_total_time=60
cargo fuzz run fuzz_contract_invocation --fuzz-dir fuzz -- -max_total_time=60
cargo fuzz run fuzz_contract_spec_parse --fuzz-dir fuzz -- -max_total_time=60
cargo fuzz run fuzz_test_generator --fuzz-dir fuzz -- -max_total_time=60

Related Issue

Closes #308

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