fix(proof): reject truncated exclusion proofs

[mablr]

Problem

proof::verify_proof wrongly accepts a truncated proof as a valid proof-of-exclusion for a key that actually exists in the trie, because an incomplete path walk causes the remaining child-hash to be filtered to None, which then matches the None expected value.

Fix

Early return error if the walked path is a strict prefix of the key and there is still a pending node or value.

Added unit test.

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

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_{Comparing mablr:fix/reject_truncated_exclusion_proofs (622ae9c) with main (e906098)}