Fix calculate_open_long negative interest check, fix get_spot_price_after_short

[ryangoree]

Description

This PR changes calculate_open_long to use the internal spot_price_after_long for it's negative interest check to ensure the governance fee is accounted for. It also updates the calculate_spot_price_after_short to take an Option<base_amount> similar to calculate_spot_price_after_long with logic to determine the shares delta, accounting for fees, if none is given.

Required for delvtech/hyperdrive-rs#33, delvtech/hyperdrive-rs#21
Related to #833
noticed while reviewing #916

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Sheng

Rust

• Testing
  • Are there new or updated unit or integration tests?
  • Do the tests cover the happy paths?
  • Do the tests cover the unhappy paths?
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    covering the full input space?
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[slundqui]

Existing tests should cover these changes. LGTM

[mcclurejt]

LGTM other than the redundant import