fix(binarycodec): verify IOU not-XRP bit before sign parsing

[e-desouza]

Why

The IOU amount deserialization function (_deserialize_issued_currency_amount) reads the sign from bit 6 of byte 0 (the 0x40 mask) but did not first verify that byte 0 has the 0x80 "not XRP" bit set, which is required by the XRPL IOU wire format. A malformed 8-byte buffer where 0x80 is absent but the buffer still reaches the IOU branch — because it is neither native (0x80 clear, 0x20 clear is native) nor MPT (0x20 set) — could trigger incorrect sign parsing. The routing invariant that currently prevents this from occurring in the Amount::serialize path is not enforced at the deserialization layer itself, leaving a gap if the routing logic is ever extended or the function is called directly.

Closes #260.

What changed

• src/core/binarycodec/types/amount.rs: added an early guard at the top of _deserialize_issued_currency_amount that returns InvalidReadFromBytesValue when bytes[0] & 0x80 == 0. The guard uses the existing _NOT_XRP_BIT_MASK constant and the same error variant already used throughout the binary codec.
• Four new unit tests in the same file:
  • test_iou_positive_sign_round_trips — a valid positive IOU (byte 0 = 0xC0) serializes without error.
  • test_iou_negative_sign_round_trips — a valid negative IOU (byte 0 = 0x80, sign bit clear) serializes with a negative value string.
  • test_iou_missing_not_xrp_bit_returns_error — bytes with 0x80 absent return an error from _deserialize_issued_currency_amount.
  • test_iou_all_zero_bytes_returns_error — all-zero bytes (also lacking 0x80) are rejected.

How to validate

cargo test --release
cargo test --release --no-default-features --features embassy-rt,core,utils,wallet,models,helpers,websocket,json-rpc
cargo clippy --features std,tokio-rt,embassy-rt,actix-rt,futures-rt,smol-rt,core,wallet,models,utils,helpers,json-rpc,websocket,cli -- -D warnings

All tests pass and clippy reports zero warnings.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.83333% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@c80d932). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/core/binarycodec/types/amount.rs	95.83%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #311   +/-   ##
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  Coverage        ?   84.27%           
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  Files           ?      200           
  Lines           ?    20802           
  Branches        ?        0           
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  Hits            ?    17531           
  Misses          ?     3271           
  Partials        ?        0           

Flag	Coverage Δ
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Files with missing lines	Coverage Δ
src/core/binarycodec/types/amount.rs	87.62% <95.83%> (ø)

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