exp/orderbook: Fix bug in CalculatePoolExpectation()

[tamirms]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've reviewed the changes in this PR and if I consider them worthwhile for being mentioned on release notes then I have updated the relevant CHANGELOG.md within the component folder structure. For example, if I changed horizon, then I updated (services/horizon/CHANGELOG.md. I add a new line item describing the change and reference to this PR. If I don't update a CHANGELOG, I acknowledge this PR's change may not be mentioned in future release notes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Fixes #5143

This commit fixes three inconsistencies with how liquidity pool payouts / expectations are calculated in core vs horizon:

1. When calculating how much we need to deposit into a pool in order to receive a desired payout we need to consider that the calculated deposit may overflow the reserves, see https://github.com/stellar/stellar-core/blob/fdd833d57c86cfe0c5057da5b2319953ab841de0/src/transactions/OfferExchange.cpp#L1361-L1363
2. We should rule out trades that will result in a payout of 0, see https://github.com/stellar/stellar-core/blob/fdd833d57c86cfe0c5057da5b2319953ab841de0/src/transactions/OfferExchange.cpp#L1325-L1327
3. Fees that are greater than or equal to 100% are illegal, see https://github.com/stellar/stellar-core/blob/fdd833d57c86cfe0c5057da5b2319953ab841de0/src/transactions/OfferExchange.cpp#L1254-L1259

(2) and (3) do not affect the output of the horizon path finding endpoint because those corner cases are excluded from consideration in the path finding algorithm.

However, (1) results in horizon returning incorrect paths which attempt to trade against liquidity pools in a way that would overflow the liquidity pool reserves. When users try to submit path payments using these incorrect paths, core is not able to execute the trades and that results in a op_too_few_offers error.

Known limitations

[N/A]

[Shaptic]

LGTM! Just a coupla comments below ⬇️ really thorough investigation and fix, though 👏