lukas/update margin calc to match onchain

[LukasDeco]

This PR updates the SDK’s margin calculation logic to:

1. Support isolated positions, and
2. Align more closely with the on-chain margin logic for improved consistency.
3. Bonus purpose: make margin and collateral calculations more efficient

Previously, the SDK would loop through user perp and spot positions multiple times, doing a lot of redundant work. With the new methods added, this PR refactors those flows so that margin is computed in a single, cleaner pass — reducing duplication and making the results better match the on-chain behavior. Through the new User.getMarginCalculation method, this is the central logic for calculating collateral and margin requirement. Now methods like User.getFreeCollateral call this function internally.

Commits:

• refactor: margin calc matches on chain func
• fix: hadnle iso balance undefined
• feat: margin calc tests

[LukasDeco]

@crispheaney does the User.calculateFreeCollateralDeltaForPerp function need to change to support isolated positions? I don't think so but I wanted to double check.

[crispheaney]

calculateFreeCollateralDeltaForPerp doesnt have to change since it doesnt look at deposits at all