fix: map Horizon upstream errors to safe stable API responses

[agba-marvel]

Overview

This PR prevents upstream Horizon error payloads from leaking through the Stellar API surface. Upstream failures are now mapped to safe, stable API errors while diagnostic context (status, operation, upstream message) is retained internally for logging/debugging.

Related Issue

Changes

🛡️ Horizon Error Mapping

• [MODIFY] BackendAcademy/src/stellar/horizon.service.ts

  • Wraps Horizon calls so upstream failures are caught and translated into safe, stable error objects instead of propagating raw Horizon payloads.
  • Retains diagnostic context (upstream status, operation, message) internally for logging, without exposing it to callers.

• [MODIFY] BackendAcademy/src/stellar/stellar.controller.ts

  • Returns the mapped, safe error responses to API consumers so upstream Horizon details are never surfaced in HTTP responses.

• [MODIFY] BackendAcademy/src/stellar/horizon.service.spec.ts

  • Adds coverage verifying upstream Horizon failures are mapped to safe stable errors and that diagnostic context is preserved internally.

Verification Results

npm test -- BackendAcademy/src/stellar/horizon.service.spec.ts
✅ Horizon error-mapping tests pass

Acceptance Criteria	Status
Upstream failures mapped to safe, stable API errors	✅ Horizon errors translated before reaching callers
Diagnostic context retained internally	✅ Upstream status/operation/message preserved for logging
Scope limited to the three allowlisted files	✅ Only horizon.service.ts, stellar.controller.ts, horizon.service.spec.ts modified

Closes #1013

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