Perps/fix available to trade balance core wt

[geositta]

Explanation

This change fixes a controller gap in @metamask/perps-controller where consumers could not distinguish between withdrawable funds and actual trading buying power for HyperLiquid accounts.

The controller only exposed availableBalance, which maps to HyperLiquid withdrawable, so clients using that field for funded state checks could incorrectly treat spot-funded accounts as unfunded even when HyperLiquid shows usable buying power.

The improvement is a new availableToTradeBalance field on perps account state. For HyperLiquid, it is calculated from perps withdrawable balance plus spot USDC, while availableBalance remains unchanged as the withdrawable only value.

The field is populated in both one-shot account fetches and live streamed account updates, and the shared account aggregation path carries it through consistently.

This gives mobile and extension a dedicated funded-state signal without changing existing withdraw behavior or order-entry calculations.

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them