test: add unit & integration tests for get_price at zero supply (#720)

[Oluwasuyi-Oluwatimilehin-Daniel]

Summary

At supply 0 (when no keys are in circulation for a creator), get_price should return the configured base_price constant — representing the price of the very first key on the bonding curve. At supply 1, get_price must return a price strictly greater than base_price when a positive slope is configured.

This PR exposes get_price on CreatorKeysContract and adds unit and integration test suites covering all boundary conditions, curve presets, and invariant checks.

Closes #720

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Changes

1. Contract Entrypoint (creator-keys/src/lib.rs)

• Added pub fn get_price(env: Env, creator: Address, supply: u64) -> Result<i128, ContractError> to CreatorKeysContract.
• Serves as a read-only bonding curve price query helper at arbitrary supply steps with overflow protection.

2. Unit Tests (creator-keys/src/test_issues.rs)

• test_get_price_at_supply_zero_returns_base_price_and_greater_at_supply_one: Verifies supply 0 returns base_price, supply 1 returns base_price + slope > base_price, and asserts neither call panics (including try_get_price).
• test_get_price_various_base_prices_at_supply_zero: Checks supply 0 and supply 1 across multiple base prices ([1, 10, 100, 500, 1000, 10000, 50000]).
• test_get_price_presets_at_supply_zero_and_one: Verifies Linear, Quadratic, and Flat curve presets at supply 0 and 1.

3. Integration & Boundary Tests (creator-keys/tests/get_price_zero_supply.rs)

• Standard Linear Curve: Confirms AC-1, AC-2, AC-3 with try-call equivalents.
• Multiple Base Prices: Verifies price calculations across wide magnitudes (from 1 to 1,000,000 stroops).
• Curve Presets: Verifies Linear, Quadratic, and Flat curve mathematics.
• Read-Only Invariant: Asserts contract state, supply, and balances are unchanged across repeated invocations.
• Quote Parity: Validates consistency between get_price, query_price, and get_buy_quote.
• Error Handling: Verifies graceful error return (ContractError::KeyPriceNotSet) when key price is uninitialized.
• Emergency Pause: Confirms get_price remains callable and answers correctly while the contract is paused.

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Acceptance Criteria Checklist

• Supply 0 returns the configured base price constant
• Supply 1 returns a price strictly above base price (when slope > 0)
• No panic in either case (both get_price and try_get_price)
• Tested across multiple base prices and presets (Linear, Quadratic, Flat)
• State is strictly read-only and unmutated

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Verification

# Integration tests
cargo test --test get_price_zero_supply
# Output: 7 passed; 0 failed; finished in 0.19s

# Unit tests
cargo test --lib test_issues::issue_tests::test_get_price
# Output: 3 passed; 0 failed; finished in 0.94s

# Full test suite
cargo test
# Output: 169 passed; 0 failed; 0 ignored

# Linters & Formatters
cargo fmt --check
cargo clippy --all-targets

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