test(contracts): bonding curve regression tests + stroop display helper

[Dannyswiss1]

This PR adds 4 regression tests and a test helper to verify bonding curve math, admin invariants, and improve test readability per audit follow-ups.

What's Changed

Buy Quote Consistency - #352

• Added test_buy_quote_updates_after_sell in contracts/src/test.rs
• Reads buy quote at supply S, performs sell to reduce supply to S'
• Asserts new buy quote matches bonding_curve_price(S') helper
• Fails if quote doesn't update after supply change
• Confirms supply changes immediately reflect in pricing

Admin Address Invariant - #353

• Added test_admin_unchanged_after_fee_update
• Records get_protocol_admin() before update_fee_config()
• Executes fee config update with new fee bps
• Asserts admin address identical after update using read-only getter
• Fails if admin field is overwritten by config logic

Stroop Display Helper - #354

• Added test_helpers.rs with to_display_units(stroops: u128) -> Decimal
• Defined const STROOP_SCALE: u128 = 10_000_000 as named constant
• Converts stroops to XLM display units for readable assertions
• Updated 2 existing tests: test_large_buy_amount and test_fee_calculation
• Before: assert_eq!(amount, 15000000000u128)
• After: assert_eq!(to_display_units(amount), dec!(1500.0))

Buy/Sell Symmetry - #355

• Added test_buy_sell_symmetric_price_impact
• Records buy price for N keys at supply S
• Buys N keys, then sells N keys
• Asserts sell_proceeds = buy_cost - fees within 1 stroop tolerance
• Documents expected relationship: sell_quote(buy(S, N)) ≈ buy_cost * (1 - fee_bps/10000)
• Confirms bonding curve behaves symmetrically minus protocol fees

Testing Done

• cargo test -p accesslayer-contracts → 4 new tests pass, 0 regressions
• cargo fmt --all -- --check → pass
• cargo clippy --all-targets -- -D warnings → 0 new warnings
• Verified test_buy_quote_updates_after_sell fails if quote logic commented out
• Verified test_admin_unchanged_after_fee_update fails if admin overwritten in update path
• Helper correctly converts: to_display_units(10_000_000) == dec!(1.0)

Notes

All tests use existing bonding_curve_price helper for expected values. No changes to contract logic, tests only. STROOP_SCALE matches Stellar asset precision.

Closes #352
Closes #353
Closes #354
Closes #355

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