test: add integration tests for bonding curve ten-buy price monotonicity

[unlimitedengineer]

Closes #697

Summary

• Adds creator-keys/tests/bonding_curve_ten_buy_monotonicity.rs with 3 integration tests covering all four acceptance criteria for issue Add integration test confirming the bonding curve price increases monotonically with each successive buy #697.
• Deploys the contract, configures a linear bonding curve (base_price = 5000, slope = 100), and registers a creator.
• Records get_buy_quote price at supply 0 and asserts it equals the configured base price (AC-3).
• Executes ten sequential single-key buys, recording the quote price before each buy, and asserts every successive price is strictly greater than the previous (AC-1 and AC-4).
• After ten buys, executes one sell and asserts the resulting buy quote price is strictly less than the price immediately before the sell (AC-2).
• Regression guard test with a flat (zero-slope) curve confirms prices do not strictly increase, proving the main test is discriminating.
• Formula verification test asserts each of the ten buy-quote prices matches base_price + slope * supply exactly.

Test plan

• test_ten_sequential_buys_price_strictly_increases_and_sell_decreases passes
• test_flat_curve_prices_do_not_strictly_increase_across_ten_buys passes
• test_ten_buy_prices_match_bonding_curve_formula passes

[Chucks1093]

The regression guard with the flat curve is a really good addition. Without it you could have a test that passes trivially if the monotonicity assertion was accidentally always true. Having a test that is designed to fail with a zero slope curve proves the main assertion is actually doing meaningful work.