feat: add sliding-window rate limiting to wallet transaction execution

[Cjay-Cyber-2]

Description — what to implement and the why/impact.

Implement a sliding-window rate limiting policy in the wallet contract (contracts/wallet). Autonomous AI agents executing transactions via wallets need protection against sudden anomalous spikes or runaway loops that could drain funds within a short interval. Adding a sliding-window rate limiter directly into the wallet execution path provides on-chain velocity checks.

Context & Requirements — background, constraints, design references, edge cases;

• Grounded in contracts/wallet using Soroban ledger timestamps (env.ledger().timestamp()).
• Must track transaction counts or cumulative values over configured time windows.
• Must return a deterministic error such as RATE_LIMIT_EXCEEDED when transactions exceed the threshold within the active window.
• Must optimize storage writes to prune expired window entries and avoid state bloat.

Acceptance Criteria — a checklist ("- [ ] ...") of specific, testable conditions that define "done".

• Define rate-limit configuration and history storage structures in contracts/wallet.
• Implement window validation logic that inspects recent ledger timestamps prior to permitting wallet execution.
• Define and export the RATE_LIMIT_EXCEEDED error constant.
• Add unit tests simulating rapid transaction attempts and verifying rate-limit enforcement across window boundaries.

Implementation Guidance — likely files/modules to touch and a suggested approach

• Likely files: contracts/wallet/src/lib.rs, contracts/wallet/src/policy.rs (or wallet transaction handler).
• Approach: Store timestamps of recent successful transactions in a bounded vector or state map. On each execute call, filter out timestamps older than the window duration and check the count or total against configured limits.

Testing & Validation — how the contributor should prove it works

• Run cargo test --package astroid-wallet to validate rate-limiting edge cases under simulated ledger time increments.
• Verify formatting and clippy checks pass cleanly across the workspace.

Submission Guidelines — must open a PR that includes "Closes #"; assignment is required before starting; follow the repo's existing style and conventions.

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Wave complexity: Medium

[Riel101]

@Riel101 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

kindly assign

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Congratulations, @Riel101! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

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