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

[gracious01-tech]

Summary

Replaces both .expect("overflow") calls in scout_access/src/lib.rs with .ok_or(ScoutAccessError::Overflow)? so overflow conditions surface as a typed contract error instead of causing a runtime panic.

Changes

contracts/scout_access/src/lib.rs

subscribe — timestamp addition:

// before
expires_at: now.checked_add(config.sub_duration_secs).expect("overflow"),

// after
expires_at: now.checked_add(config.sub_duration_secs).ok_or(ScoutAccessError::Overflow)?,

log_trial_offer — trial counter increment:

// before
let next_index = index.checked_add(1).expect("overflow");

// after
let next_index = index.checked_add(1).ok_or(ScoutAccessError::Overflow)?;

Both functions already return Result<_, ScoutAccessError> so the ? operator propagates cleanly with no signature changes.

New test — test_log_trial_offer_overflow_returns_error

Seeds DataKey::TrialCounter(1) at u32::MAX via env.as_contract, then calls try_log_trial_offer and asserts the result is Err(Ok(ScoutAccessError::Overflow)). No fee is collected and no state is written.

What was not changed

• No function signatures changed
• No other production logic touched
• All existing tests continue to pass unmodified