fix(binarycodec): guard NO_ACCOUNT issuer collision in Issue parser

[e-desouza]

Summary

• Adds an explicit error in Issue::from_parser when an IOU's issuer bytes equal the NO_ACCOUNT sentinel (rrrrrrrrrrrrrrrrrrrrBZbvji) and the currency field starts with 0x00 (standard IOU currency layout). Without this guard, the parser silently misclassifies such an IOU as an MPT issue and consumes four extra bytes from the parser stream, corrupting everything that follows (Issue NO_ACCOUNT sentinel collides with legitimate AccountID #256).
• Clarifies in comments that the MPT Issue wire format stores the sequence field in little-endian byte order, matching what the codec-fixture test suite expects. The original LE storage was correct; the new comments make this explicit rather than leaving it implicit (Issue MPT sequence stored in LE inside a BE-elsewhere codec #255).
• Adds two new tests: test_issue_mpt_sequence_roundtrip_endian (JSON -> binary -> JSON survives for a known MPT issuance ID, verifying the LE storage) and test_issue_from_parser_no_account_issuer_collision_returns_error (USD + NO_ACCOUNT issuer returns an explicit error instead of silently misparsing).

What changed

• src/core/binarycodec/types/issue.rs: expanded NO_ACCOUNT constant doc comment; added byte[0] guard in from_parser; clarified LE comment in TryFrom<Value> and Serialize.
• src/core/binarycodec/test/tx_encode_decode_tests.rs: two new tests for the MPT sequence round-trip and the colliding-issuer error path.

How to validate

cargo test --release
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

Both pass with zero failures and zero warnings.

Closes #255, closes #256

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #312   +/-   ##
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  Coverage        ?   84.24%           
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  Files           ?      200           
  Lines           ?    20752           
  Branches        ?        0           
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  Hits            ?    17483           
  Misses          ?     3269           
  Partials        ?        0           

Flag	Coverage Δ
unit	84.24% <100.00%> (?)

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Files with missing lines	Coverage Δ
src/core/binarycodec/types/issue.rs	86.73% <100.00%> (ø)

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[e-desouza]

Follow-up to second opinion review: the bytes[0] == 0x00 heuristic added to guard against the NO_ACCOUNT issuer collision was incorrect. MPT issuer account hashes are raw 20-byte hashes and can legitimately start with 0x00 (roughly 1/256 of accounts), so the guard would reject valid MPT issues. Pushed 571f183 which removes the guard and reverts to treating NO_ACCOUNT as an unambiguous MPT type tag. The XRPL ledger itself prevents any account from holding the address rrrrrrrrrrrrrrrrrrrrBZbvji, so no valid IOU issuer can equal NO_ACCOUNT in a well-formed stream.