Revert "fix(scout_access): replace expect("overflow") with ScoutAccessError::Overflow"

[gracious01-tech]

Description

Replaced panic-based overflow handling in log_trial_offer with a typed contract error.

Previously, the contract used .expect("overflow") on checked_add, which caused execution to panic instead of returning a predictable contract error.

This update now safely returns ScoutAccessError::Overflow when the counter exceeds the maximum u32 limit.

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Related Issue

Closes scout-off#109

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Type of Change

• Bug fix
• Smart contract reliability improvement
• Error handling enhancement
• New feature
• Refactor
• Documentation

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Changes Included

• Removed .expect("overflow") from scout_access/src/lib.rs
• Added graceful overflow handling using ScoutAccessError::Overflow
• Updated counter increment logic to return typed errors
• Added overflow boundary unit tests

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Security Impact

Improves contract stability and prevents panic-based execution failures.

No authorization or payment logic changed.

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Testing

• Added unit test simulating overflow using u32::MAX
• Verified ScoutAccessError::Overflow is returned correctly
• Verified normal counter increments still succeed
• Existing tests remain green

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Manual Testing Steps

• Set TrialCounter to u32::MAX
• Called log_trial_offer
• Verified contract returns ScoutAccessError::Overflow
• Confirmed no panic occurs
• Tested standard increment flow successfully

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Acceptance Criteria Coverage

• No .expect("overflow") remains in scout_access/src/lib.rs
• Unit test simulates overflow using u32::MAX

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Breaking Changes

No breaking changes.

This change only replaces panic behavior with typed error handling.

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Checklist

• Code builds successfully
• Tests pass
• Follows project conventions
• No sensitive data exposed
• Existing functionality preserved

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Additional Context

Using typed errors instead of panics improves predictability for integrators and aligns with recommended Soroban smart contract error-handling practices.

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Reviewer Notes

Please focus on:

• Correct overflow detection logic
• Proper propagation of ScoutAccessError::Overflow
• Removal of panic-based handling
• Boundary-condition test coverage