[CO-07] Fix off-by-one expiry in loyalty_token approve() causing immediate silent expiration

[Leothosine]

Problem

contracts/loyalty_token/src/lib.rs has inconsistent boundary conditions between approve() and get_allowance():

• approve() rejects only if expiration_ledger < env.ledger().sequence() (strict less-than)
• get_allowance() returns 0 when env.ledger().sequence() > d.expiration_ledger (strict greater-than)

A caller who sets expiration_ledger = current_sequence + 1 gets approval valid for only ~5 seconds (one ledger). There is no minimum validity window enforced, no maximum cap, and the semantics of expiration_ledger are undocumented - making it easy for integrators to create approvals that expire before they can be used.

Proposed Solution

1. Add MIN_APPROVAL_VALIDITY_LEDGERS: u32 = 17280 constant (~24 hours at 5s/ledger)
2. Change approve() to require expiration_ledger >= env.ledger().sequence() + MIN_APPROVAL_VALIDITY_LEDGERS
3. Add a MAX_APPROVAL_VALIDITY_LEDGERS cap (~1 year) to prevent indefinite approvals
4. Document in code comments: expiration_ledger is the LAST ledger at which the approval is valid (inclusive)

Acceptance Criteria

• Approving with expiration_ledger < current + MIN_APPROVAL_VALIDITY_LEDGERS panics with "expiration too soon"
• A valid approval is usable up to and including expiration_ledger
• A valid approval returns 0 on expiration_ledger + 1
• Unit test matrix covers: too-short expiry (panics), minimum valid expiry, at-expiry-ledger (valid), past-expiry (returns 0)
• Code comment documents the inclusive expiry semantics

[funds0033-cmyk]

pls i would like to work on this i have the skillset to deliver your request flawlessly kindly assign it to me

[Senorespecial]

I would really like to work on this issue, please assign.

[bellabuks]

Please can i work on this issue. I will deliver beyond expectations. ETA is 1 day/ A well structured, modular and maintainable pr.

[grantfox-oss]

🦊 GrantFox — @bellabuks has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #285)
2. Your PR will be reviewed by the tosirano maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@bellabuks's PR #291 was approved and merged by @Leothosine.

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👏 Great work, @bellabuks! Keep contributing to tosirano.