fix: distinguish Horizon account lookup failures

[ayaanoncrypto]

Summary

Fixes #913

The destination-account check in buildPaymentTransaction now distinguishes an account that does not exist from a transient Horizon failure.

Changes

• Treat Horizon 404 responses and not_found problem types as an unfunded destination.
• • Preserve the create-account path only for genuine missing accounts.
• • Convert rate limits, 5xx responses, timeouts, and network failures into the clear retryable error Horizon unavailable, please retry.
• • Add typed error handling without changing the existing transaction construction flow.
• • Add regression coverage for nested and top-level 404 responses, not_found, transient 500 and timeout errors, retryable messaging, and cause preservation.

Testing

• git diff --check passed.
• • Added five focused Jest regression tests.
• • Full npm test, lint, and type-check were not run because the filtered checkout has no node_modules directory.

[github-actions]

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