[Backend] — Test: `PredictionsService.submit` never calls Soroban when market validation fails

[Olowodarey]

Background

PredictionsService.submit (src/predictions/predictions.service.ts) validates
the market state (closed, cancelled, expired) before calling
this.sorobanService.submitPrediction. If the Soroban call is reached on an
invalid market, real XLM gets locked on-chain without a corresponding DB row.
There are currently no unit tests confirming the short-circuit behavior — only
the happy path is covered.

Goal

Add unit tests in src/predictions/predictions.service.spec.ts confirming that
SorobanService.submitPrediction is never called when market validation
fails.

Requirements

Create a predictions.service.spec.ts (or extend existing) with the following
cases. Use jest.spyOn(sorobanService, 'submitPrediction') to assert it is not
called.

1. Market not found: marketsRepository.findOne returns null →...

[gloskull]

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