fix: apply max_slip_fee cap in buy-side inversion

[F3Joule]

Description

Fixes a bug where max_slip_fee cap was not applied during buy-side trade inversion in the
Omnipool math, breaking the invariant delta_hub_reserve_in − total_protocol_fee = D_net.

Changes:

• calculate_buy_state_changes — after invert_buy_side_slip, check if cap fired and override
  d_gross with the capped linear formula; same check after invert_sell_side_fees for the
  sell-side leg of buy inversion
• Add regression tests buy_slip_fee_cap_invariant_holds and sell_slip_fee_cap_invariant_holds

Related Issue

#1412

Motivation and Context

The forward path (calculate_slip_fee_amount) correctly caps the slip fee; the inverse path did
not - so the two paths produced inconsistent results when the slip rate exceeded max_slip_fee.
Sell trades are unaffected (no inversion involved).

How Has This Been Tested?

Two regression tests, one per inversion step, each verified to fail before the fix and pass after.

Checklist:

• I have updated the documentation if necessary.
• I have added tests to cover my changes, regression test if fixing an issue.
• This is a breaking change.

[github-actions]

Crate versions that have not been updated:

• hydra-dx-math: v13.2.1

Runtime version has not been increased.

[github-actions]

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