refactor(contract): extract daily-reset check into shared helper with boundary tests

[leocagli]

Fixes #50

Problem

The daily-spend reset condition (now >= last_reset_ledger + DAY_LEDGERS) was inlined independently in four places — get_policy, check_spending_allowed, record_payment, and update_policy — with no shared abstraction and zero test coverage. Any future change to the reset threshold or semantics (e.g. moving to wall-clock time, adding a grace period) would require updating all four sites with no safety net.

Solution

Extracted helper:

fn is_new_day(now: u64, last_reset_ledger: u64) -> bool {
    now >= last_reset_ledger + DAY_LEDGERS
}

Replaced all four inline occurrences with is_new_day(now, policy.last_reset_ledger) / is_new_day(now, p.last_reset_ledger).

Added #[cfg(test)] module with 5 boundary tests:

Test	now	Expected
One ledger before threshold	last_reset + DAY_LEDGERS - 1	false (no reset)
Exactly at threshold	last_reset + DAY_LEDGERS	true (reset)
One ledger past threshold	last_reset + DAY_LEDGERS + 1	true (reset)
Same ledger as reset	last_reset	false (no reset)
Multiple days past threshold	last_reset + DAY_LEDGERS * 3	true (reset)

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Reviewing files that changed from the base of the PR and between 6fb6da9 and 763eedc.

📒 Files selected for processing (1)

• contract/agents/src/lib.rs

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