fix(drive): add bounds check for i64 cast in token balance addition

[QuantumExplorer]

Summary

• Add i64 bounds check before cast in token balance checked_add path
• The insert path already has this check but the add path was missing it

Issue

Security audit found that balance_to_add as i64 in the add-to-existing-balance path has no bounds check. Values above i64::MAX silently wrap to negative, potentially producing incorrect balance results. The insert path correctly checks balance_to_add > i64::MAX as u64.

Test plan

• Test verifies error on balance_to_add > i64::MAX
• Test verifies normal additions work
• cargo test passes
• cargo clippy clean

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 1c8502fb-ddee-4bc1-abf7-20eca22ef7ca

📥 Commits

Reviewing files that changed from the base of the PR and between 658d02d and 591213f.

📒 Files selected for processing (1)

• packages/rs-drive/src/drive/tokens/balance/add_to_previous_token_balance/v0/mod.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/token-balance-i64-truncation

📝 Coding Plan

• Generate coding plan for human review comments

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[codecov]

Codecov Report

❌ Patch coverage is 94.50549% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.32%. Comparing base (658d02d) to head (591213f).
⚠️ Report is 21 commits behind head on v3.1-dev.

Files with missing lines	Patch %	Lines
...ns/balance/add_to_previous_token_balance/v0/mod.rs	94.50%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           v3.1-dev    #3295      +/-   ##
============================================
- Coverage     70.42%   68.32%   -2.10%     
============================================
  Files          3293     3293              
  Lines        262598   262689      +91     
============================================
- Hits         184931   179487    -5444     
- Misses        77667    83202    +5535     

Components	Coverage Δ
dpp	56.61% <ø> (-1.82%)	⬇️
drive	74.88% <94.50%> (-3.32%)	⬇️
drive-abci	81.44% <ø> (-1.49%)	⬇️
sdk	30.34% <ø> (-0.91%)	⬇️
dapi-client	39.08% <ø> (ø)
platform-version	∅ <ø> (∅)
platform-value	39.35% <ø> (ø)
platform-wallet	60.40% <ø> (ø)
drive-proof-verifier	∅ <ø> (∅)

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[QuantumExplorer]

Sure