CONTRACT: Add proposal expiry guard & test

[OSEH-svg]

Description
Multi‑sig proposals can be approved after their expires_at timestamp, which is a security issue.

Goal
Reject any approval attempt on an expired proposal before modifying the approvals list.

Tasks

1. In the proposal approve handler, fetch the proposal and immediately check if env.ledger().timestamp() > proposal.expires_at { return Err(EventError::ProposalExpired) }.
2. Add a new error variant ProposalExpired.
3. Write a Rust test that creates a proposal with expires_at in the past, then attempts to approve it and expects ProposalExpired.

Acceptance Criteria

• Approving an expired proposal returns ProposalExpired and does not alter the approvals list.
• The error appears in the contract API docs (Feat/add created at and updated at fields( issues 570 574) #975).

Dependencies
Depends on: None
Blocks: None

[Abba073]

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Hello Sir,

I came across your project and will love to have the opportunity to work on it

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[khanavi272-spec]

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I would like to work on this issue

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[owohdora-stack]

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Good day maintainer i can handle this quickly please kindly assign to me

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[Otaiki1]

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Allow me work on this . PR in less than 24 hours

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