fix calculate_spot_price_after_short and add open short deltas & preview

[dpaiton]

Resolved Issues

No issue

Description

calculate_spot_price_after_short did not correctly compute the share deltas when a base_amount was not given. This PR adds a test to demonstrate this as well as a fix.

Part of the process also required adding calculate_open_short_deltas and a corresponding test.

Since I was most of the way there, I also added a preview_open_short that returns a mutated state after the reserve deltas have been applied.

Review Checklists

Please check each item before approving the pull request. While going
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• Testing
  • Are there new or updated unit or integration tests?
  • Do the tests cover the happy paths?
  • Do the tests cover the unhappy paths?
  • Are there an adequate number of fuzz tests to ensure that we are
    covering the full input space?
  • If matching Solidity behavior, are there differential fuzz tests that
    ensure that Rust matches Solidity?

[sentilesdal]

overall looks good! I think we should get in the habit of adding /// comments at the top of functions to give clues what they do, helps reviewers at least.