feat: add structured balance errors for Stellar fee computation

[Julink-eth]

Explanation

Expose structured balance errors when Stellar fee computation fails due to insufficient funds.

• Add and export the NonEvmBalanceError type.
• Validate balance errors returned directly or through a nested Snap error.
• Attach valid errors to quote metadata as nonEvmBalanceError.
• Preserve existing behavior for non-balance and non-Stellar errors.

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

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Note

Low Risk
Additive, Stellar-only error handling on the non-EVM fee catch path; success paths and other chains are unchanged.

Overview
When Stellar non-EVM fee computation via the wallet snap fails for insufficient funds, clients can now show specific balance messaging instead of a generic missing-fee state.

The PR introduces and exports NonEvmBalanceError (codes InsufficientBalance / InsufficientBalanceToCoverFee, asset id, available/required amounts, optional reserveAmount for on-chain minimums). That payload is attached on quotes as nonEvmBalanceError on NonEvmFees and QuoteMetadata.

In appendNonEvmFees, only Stellar source quotes parse snap rejections: validators read structured data from error.data or nested error.data.cause.data, ignore malformed payloads, and otherwise keep the existing log-and-undefined-fee behavior for other chains and non-balance failures. Unit tests cover happy paths and validation edge cases.

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