[CONTRACTS] Add InvalidExpiration error for past expires_at on add

[wheval]

Summary

Contract InvalidExpiration error when expires_at is in the past on add_session_key with SDK mapping and tests.

Background

Past expiry must fail at contract level consistently; medium adds error enum, unit tests, SDK error code, and doc update.

This is a medium task: expect multiple files, tests, and integration with existing flows—not a single-line or docs-only change.

Scope

• contracts/account/src/lib.rs
• contracts/account/src/test.rs
• packages/core-sdk/src/errors.ts

Files to create / modify

Define error; guard in add path; contract tests; map in SDK when simulation returns error.

Example implementation

#[contracterror]
pub enum Error {
  InvalidExpiration = 12,
}

Acceptance criteria

• Adding key with past ledger time fails with InvalidExpiration
• Contract tests cover boundary (now vs now-1)
• SDK surfaces typed error to wallets
• docs/contract-methods.md updated
• Snapshot test updated if applicable

Difficulty

Medium

Labels

enhancement, medium, contract

[BigMick03]

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Please assign this to me, I can work on this. please.

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[Itodo-S]

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please let me work on this issue; I can handle it.

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[Daveside9]

@Daveside9 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

can i do this? merge in 2hours.

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[numdinkushi]

@numdinkushi has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi, I can handle this. Kindly assign to me

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[sshdopey]

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I'd love to work on this.

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Congratulations, @sshdopey! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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