Remittance Split: Add DeadlineExpired/InvalidDeadline boundary tests for distribute_usdc_signed

[Baskarayelu]

Description

The signed distribution path uses a deadline embedded in DistributeUsdcRequest and returns RemittanceSplitError::DeadlineExpired (14) or InvalidDeadline (25) when the deadline is malformed or in the past. Boundary behavior at deadline == now, deadline == now - 1, and deadlines beyond SIGNATURE_EXPIRATION must be deterministically tested, since off-by-one errors here directly affect replay-window safety. There are no dedicated boundary tests asserting the exact comparison semantics.

Requirements and context

• Secure: Deadline comparison must be checked against env.ledger().timestamp() before require_auth side effects; expired requests must not advance the nonce.
• Tested: Equal-to-now (accepted or rejected per spec), one-second-past (rejected), far-future beyond SIGNATURE_EXPIRATION (rejected as InvalidDeadline).
• Documented: Document the deadline window semantics.
• Reference real symbols: distribute_usdc_signed, DistributeUsdcRequest, RemittanceSplitError::{DeadlineExpired, InvalidDeadline}, SIGNATURE_EXPIRATION, compute_request_hash.

Suggested execution

• Branch feature/rs-deadline-boundary-tests.
• Confirm comparison operators in remittance_split/src/lib.rs; add boundary tests in remittance_split/src/test.rs.
• Add docs docs/remittance-split-deadline-window.md.
• Add /// comments clarifying inclusive/exclusive bounds.
• Validate security assumptions: no nonce advance on expired/invalid deadline.

Test and commit

• Run cargo test -p remittance_split.
• Cover edge cases: now, now-1, now + SIGNATURE_EXPIRATION + 1.
• Include test output and replay-window security note.

Example commit message

test: add DeadlineExpired/InvalidDeadline boundary coverage for signed distribution

Guidelines

• Minimum 95% test coverage
• Clear documentation in docs/ and inline /// comments
• Timeframe: 96 hours

[Gutopro]

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