[isaacsamual994-lang]

Summary

• Add regression tests for missing Stellar accounts, Horizon HTTP 400/429/500/503 failures, transport failures, malformed JSON, missing native balances, and correct native-balance selection.
• Strengthen wallet-signing failure assertions to require actionable user-facing error messages even when the underlying wallet rejects unexpectedly.
• Fix getAccountDetails so Horizon 404 retains STELLAR_ACCOUNT_NOT_FOUND instead of being incorrectly converted to a generic fetch failure; await JSON parsing so malformed Horizon responses are normalized as STELLAR_FETCH_FAILED.

Verification

• 27/27 targeted tests passed with Vitest 4.1.10 on a macOS isolated test setup using copies of the exact changed source/test files and a stub of the wallet SDK module.
• git diff --check passed.
• Full application CI has not run yet because GitHub Actions marks fork PR workflows as action_required pending upstream approval.

Refs #56

No paid bounty/reward is assumed. The issue is already assigned to a different contributor.