[CONTRACT] Implement temporary-storage idempotency windows for external triggers

[dinahmaccodes]

Tier: 🔴 Hard
Tags: contract, hard, soroban-rust, storage, idempotency

Description:

Use temporary storage for short-lived idempotency windows keyed by action hash to prevent accidental duplicate processing.

Tasks:

• Add temporary-storage key schema for idempotency
• Enforce duplicate rejection within configured window
• Add tests for first-run and replay behavior

Acceptance Criteria:

• Duplicate actions in-window are rejected
• Keys expire naturally without persistent rent overhead

[ussyalfaks]

@ussyalfaks has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

i would like to work on this issue

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[CHEF-SAVY]

@CHEF-SAVY has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Gm gm. My name's Isaac. I can work on this issue and i would appreciate it if i can be assigned to this issue. please assign.

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This issue has been completed by @CHEF-SAVY as part of the Stellar Wave Program's 3rd Wave 🥳

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