Implement multi-signature stream creation requiring approval from a second authorised address

[Chuks-coderr]

Description

Add a two-of-two multisig creation flow where the sender proposes a stream and a designated co-signer must approve it within a configurable ledger window before the stream activates. Proposals that expire without co-signer approval are automatically discarded and the deposit is returned.

Acceptance Criteria

• Feature is fully implemented
• Tests pass
• Lint/type checks pass
• PR references this issue number

Notes

⚠️ Do not begin coding until you have been assigned by the maintainer.
Branch as feat/470-contract-multisig-creation where N is this issue number.

[kuse-design]

@kuse-design has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

i would like to work on this issue because am a software engineer and am capable of completing this task effectively

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[Calebstack]

@Calebstack has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello maintainer, i’ve got this covered! Ready to drop a high-impact, clean PR that gets this merged seamlessly

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[flexocode442]

@flexocode442 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello, I’m interested in working on this issue and would appreciate being assigned to it. Thank you.

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[sanctus-mvp]

@sanctus-mvp has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer, I’d love to work on this issue. I have relevant experience in this area and can deliver a reliable, high-quality solution.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @sanctus-mvp to this issue.