[Trovic1]

This PR adds comprehensive unit tests for the buy slippage protection guard in \creator-keys.

Covered Scenarios:

• **Actual cost equal to \max_price**: Succeeds as expected.
• Actual cost exceeding \max_price\ by 1 stroop: Reverts with \ContractError::SlippageExceeded.
• \max_price\ of \u128::MAX\ (\i128::MAX\ max boundary): Never triggers the slippage guard and succeeds.
• \max_price\ set to 0: Reverts with \ContractError::SlippageExceeded\ unless the key is free (base price = 0), in which case it succeeds.
• No state mutation on panic: Verifies state snapshot remains unchanged when the guard panics.

Closes #676

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