Add RateLimiter Token-Bucket invariant tests with deterministic ledger clock

[greatest0fallt1me]

Description

rate_limiter.rs implements a per-user limiter, but the refill arithmetic is not invariant-tested under fast-forwarded ledger clocks. Add a property test that drives env.ledger().set and asserts tokens never exceed bucket capacity.

Requirements and Context

• Fast-forward ledger sequence and timestamp deterministically
• Assert tokens <= capacity after any sequence of acquire/refill operations
• Detect potential underflow on refill
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b test/rate-limiter-bucket-invariants

2. Implement changes
   • contracts/predictify-hybrid/tests/rate_limiter_invariants.rs (new)
   • contracts/predictify-hybrid/src/rate_limiter.rs (only on failure)
3. Test and commit
   • cargo test -p predictify-hybrid rate_limiter_invariants -- --nocapture
   • Cover edge cases: time goes backward (rejected), very large dt, zero refill rate
   • Include test output and notes in the PR

Example commit message

test: add token-bucket invariants for RateLimiter

Acceptance Criteria

• Invariant holds under 1000+ random schedules
• Backward-time scenarios rejected
• Documented in rate_limiter.rs
• No unwrap() added

Guidelines

• Use env.ledger().set_timestamp only inside tests
• Minimum 95% coverage
• Clear documentation and inline comments
• Timeframe: 96 hours

[karanjakevin39-collab]

@karanjakevin39-collab has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer can i work on this issue ?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @karanjakevin39-collab to this issue.

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #620)
2. Your PR will be reviewed by the Predictify-org maintainers

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Congratulations, @karanjakevin39-collab! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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