slippage protection to the tip entrypoint added - #1430

Description

If min-tip or fee config changes between a user signing a tip and the transaction landing, the on-chain outcome can differ from what the UI showed. send_tip now accepts optional expected_min_tip and expected_fee_bps so callers can pin those values. A mismatch returns ConfigMismatch before any storage mutation or event. Passing None for both skips the checks and preserves prior tip behaviour.

Closes #1165

Type of Change

Please mark the options that are relevant:

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 🧪 Tests (adding new tests or updating existing tests)
• 📝 Documentation (changes to documentation/configs only)
• 🚀 DevOps & CI/CD (changes to workflows, scripts, or templates)

Changes Made

• contracts/tipz/src/lib.rs, tips.rs: send_tip takes optional expected_min_tip / expected_fee_bps; mismatch aborts with ConfigMismatch before TTL, rate-limit, transfer, or events.
• contracts/tipz/src/errors.rs: add ConfigMismatch = 50 (Soroban error-enum cap is 50; RefundRequestExpired is now an alias of InvalidInput).
• contracts/tipz/src/subscription.rs: recurring tips pass None/None so existing behaviour is unchanged.
• contracts/tipz/src/test/test_tip_expectations.rs: tests for matching pins, mismatched min, mismatched fee, and omitted params.
• frontend-scaffold/src/hooks/useContract.ts: sendTip encodes the new optional args (defaults to None).

How to Test

1. From contracts/tipz, run: cargo test tip_expectations -- --test-threads=1
2. Confirm these four tests pass:
   • test_send_tip_matching_expectations
   • test_send_tip_mismatched_min_tip
   • test_send_tip_mismatched_fee_bps
   • test_send_tip_omitted_expectations
3. Optionally send a tip with None/None and confirm it still credits the creator as before. Send again with a stale expected_min_tip or expected_fee_bps and confirm ConfigMismatch with no balance or event change.

Checklist

💻 Smart Contract Changes (if applicable)

• Running cargo fmt -- --check passes successfully.
• Running cargo clippy -- -D warnings runs without any warnings.
• All tests pass successfully using cargo test.
• New unit or integration tests have been written to cover the changes.
• No hardcoded values are present (e.g. addresses, fees) that should be configurable.

🎨 Frontend Changes (if applicable)

• TypeScript compiles cleanly with no errors (npm run typecheck or npx tsc --noEmit).
• Running npm run lint shows no linting errors.
• The production build compiles successfully via npm run build.
• Changes verified on local browser environment with Freighter/xBull/Albedo wallet.
• Responsive design verified (tested on mobile, tablet, and desktop viewport sizes).
• Keyboard navigation and accessibility (a11y) considerations are addressed.

⚙️ General

• Code follows the project's coding standards and structure guidelines.
• Self-reviewed the changes to ensure clean code with no commented-out code blocks.
• No console.log or debug code remains in production files.
• The branch is up-to-date with the main branch.

Screenshots / Demos (if applicable)

N/A — contract ABI / logic change only.