fix(helpers): prevent u8 overflow in signer fee multiplier

[e-desouza]

Why

calculate_fee_per_transaction_type computed (1 + signers_count) where signers_count: u8. At the maximum value of 255, this performs u8 addition: debug builds panic; release builds wrap silently to 0, zeroing the signer fee surcharge and producing a transaction with an incorrect fee.

is_not_later_rippled_version split the server's build_version string and accessed the resulting Vec at indices 1 and 2 without a length check. A non-semver build_version string (e.g. a dev build returning "1") panicked every autofill call.

calculate_based_on_fulfillment collected chars().map(|c| c as u8) to compute a byte length for the EscrowFinish fee formula. This silently undercounts for multi-byte codepoints and allocates unnecessarily.

Fixes #238.

What changed

• Line 178 of asynch/transaction/mod.rs: widened to 1u64 + signers_count as u64.
• is_not_later_rippled_version: added if source_decomp.len() < 3 || target_decomp.len() < 3 { return Ok(false); } before any index access.
• calculate_based_on_fulfillment: replaced chars().map(|c| c as u8).collect::<Vec<_>>().len() with fulfillment.len().

How to validate

cargo test --release

Regression test test_fee_signer_multiplier_overflow_guard verifies the production expression yields BigDecimal::from(256) for signers_count = 255.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.71%. Comparing base (c83df4f) to head (960bda8).

Files with missing lines	Patch %	Lines
src/asynch/transaction/mod.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #340   +/-   ##
=======================================
  Coverage   86.71%   86.71%           
=======================================
  Files         252      252           
  Lines       32630    32632    +2     
=======================================
+ Hits        28296    28298    +2     
  Misses       4334     4334           

Flag	Coverage Δ
integration	71.36% <50.00%> (+0.03%)	⬆️
unit	87.33% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/asynch/transaction/mod.rs	70.08% <50.00%> (+0.24%)	⬆️

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[Copilot]

Pull request overview

This PR hardens fee calculation and version parsing in the async transaction helper layer to prevent panics and incorrect fee computation under edge-case inputs.

Changes:

• Prevents u8 overflow in the signer fee multiplier by widening arithmetic before converting to BigDecimal.
• Avoids panics in is_not_later_rippled_version by adding a length check before indexing version components.
• Improves EscrowFinish fulfillment fee computation by using the string’s byte length directly (removing an unnecessary allocation), and adds a regression test for the signer multiplier overflow case.

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