feat: add validation error struct for computeFee to handle spendable balance error

[Julink-eth]

Explanation

Return structured validation errors from computeFee for insufficient native balance and fee coverage.
Report accurate available and required balances for swaps with multiple native debits.

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