fix(merk): reject overlong keys before encoding

[QuantumExplorer]

Summary

• reject raw Merk apply batches containing keys longer than 255 bytes before mutating the tree
• make Merk link encoding return an error for overlong keys instead of relying on debug assertions
• make grovedb-query proof op encoding use checked key-length conversion for every key-bearing variant

Fixes #681.

Verification

• cargo test -p grovedb-merk apply_rejects_overlong_keys --lib
• cargo test -p grovedb-merk encode_link_long_key --lib
• cargo test -p grovedb-query long_key --lib
• cargo test -p grovedb-merk apply_rejects_overlong_keys --lib --release
• cargo test -p grovedb-merk encode_link_long_key --lib --release
• cargo test -p grovedb-query long_key --lib --release
• cargo test -p grovedb-merk -p grovedb-query

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 2f9b6388-bf65-468f-9ea1-1739d96a64ed

📥 Commits

Reviewing files that changed from the base of the PR and between a2129a9 and f758271.

📒 Files selected for processing (4)

• grovedb-query/src/proofs/encoding.rs
• merk/src/merk/apply.rs
• merk/src/merk/mod.rs
• merk/src/tree/link.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/fix-681-merk-overlong-keys

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[codecov]

Codecov Report

❌ Patch coverage is 91.17647% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.44%. Comparing base (60f2968) to head (f758271).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
grovedb-query/src/proofs/encoding.rs	87.75%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #728      +/-   ##
===========================================
+ Coverage    91.42%   91.44%   +0.01%     
===========================================
  Files          236      236              
  Lines        67053    67161     +108     
===========================================
+ Hits         61305    61416     +111     
+ Misses        5748     5745       -3     

Components	Coverage Δ
grovedb-core	88.94% <ø> (ø)
merk	92.29% <100.00%> (+0.02%)	⬆️
storage	86.36% <ø> (ø)
commitment-tree	96.43% <ø> (ø)
mmr	96.79% <ø> (+0.03%)	⬆️
bulk-append-tree	89.39% <ø> (+0.13%)	⬆️
element	97.38% <ø> (+0.01%)	⬆️

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[QuantumExplorer]

Reviewed