Add bets.rs slippage guard max_fee_bps_at_bet to protect bettors from mid-bet fee changes

[greatest0fallt1me]

Description

A bettor calling place_bet today implicitly accepts whatever fee config is current. Add an optional max_fee_bps parameter that the contract checks against the live FeeConfig, reverting if the actual fee exceeds the bettor's tolerance. This mirrors EVM slippage protection.

Requirements and Context

• Parameter is Option<u32>; None preserves existing behaviour
• Revert with Error::FeeAboveAcceptable
• Add to batch path too
• 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 feature/bet-slippage-guard

2. Implement changes
   • contracts/predictify-hybrid/src/bets.rs
   • contracts/predictify-hybrid/src/lib.rs
   • contracts/predictify-hybrid/src/bet_tests.rs
3. Test and commit
   • cargo test -p predictify-hybrid bet_slippage
   • Cover edge cases
   • Include test output and notes in the PR

Example commit message

feat: add max_fee_bps slippage guard to place_bet

Acceptance Criteria

• None preserves prior behaviour for compatibility
• Live fee above max_fee_bps causes revert
• Test covers exact-equal threshold

Guidelines

• No unwrap() in production paths
• Clear documentation and inline comments
• Timeframe: 96 hours

[samuelfrancis163-eng]

@samuelfrancis163-eng has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I WOULD LOVE TO WORK ON THIS ISSUE BECAUSE I AM CONFIDENT THAT I CAN DELIVER EFFICIENTLY
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Congratulations, @samuelfrancis163-eng! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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