Add integration test for the price preview endpoint returning the correct buy cost for a given quantity at the current supply

[Chucks1093]

Summary

The price preview endpoint lets users see the XLM cost of buying N keys before committing. An integration test should confirm the preview returns the correct cost at different supply levels and that it is consistent with what the buy endpoint actually charges.

Scope

• Seed a creator with supply 10 and query the preview for buying 5 keys
• Assert the returned cost matches the bonding curve formula for supply 10, amount 5
• Seed a creator with supply 50 and assert the cost is higher than at supply 10 for the same amount
• Assert the preview does not mutate any server or contract state
• Assert the response includes gross_cost, fee, and net_cost fields

Acceptance Criteria

• Correct cost returned for supply 10, amount 5
• Higher supply returns higher cost for the same amount
• No state mutated by calling the preview endpoint
• Response includes gross_cost, fee, and net_cost

ETA: 12 hours

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Coordinate on Telegram

[GazzyLee]

heelo boss, I'd like to take this on. I'll add integration tests for the price preview endpoint, verify the bonding curve calculations across different supply levels, confirm the preview is read-only, and validate the gross_cost, fee, and net_cost response fields. ETA is in 8 hours tops, please assign