fuzz: Bound package eval tx fanout #35318

Fixes #35207.

The tx_package_eval fuzz target intentionally creates multi-transaction packages, including packages over the package-count limit. However, it does not need each generated transaction's input and output counts to scale with the reusable mempool outpoint set. After the ancestor and descendant size limits were removed from mempool limits, that can make some generated inputs spend a large amount of time in set churn rather than package evaluation.

This bounds the per-transaction input/output fanout generated by the harness while keeping the existing 1-to-26 transaction package range. The final package transaction still spends all in-package outputs, so the target continues to exercise package evaluation paths and the package-count boundary.

Testing:

cmake --build build_fuzz_nosan --target fuzz -j2
FUZZ=tx_package_eval build_fuzz_nosan/bin/fuzz -runs=10000
cmake --build build_fuzz --target fuzz -j2
ASAN_OPTIONS=detect_leaks=0 FUZZ=tx_package_eval build_fuzz/bin/fuzz -runs=1000

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35318.

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