test(backend): add Soroban E2E contract flow tests (#533)

[shadrach68]

closes #533

Summary

Resolves #533 (BE-56).

This PR introduces a gated end-to-end test suite that executes real Soroban contract flows against the Stellar Testnet. It validates the full lifecycle (deposit → backend indexer polling → refund) to ensure contract deployments and backend indexers function properly together and to prevent future regressions.

Changes Made

• Gated CI Pipeline: Added .github/workflows/soroban-e2e.yml configured to run only on workflow_dispatch (on-demand) or pushes to the protected main branch. This secures the E2E_WALLET_SECRET.
• E2E Test Runner: Implemented app/backend/test/soroban-flow.e2e-spec.ts which uses @stellar/stellar-sdk and NestJS utilities to directly invoke the Soroban RPC.
• Network Validation: Added strict checks to ensure the network passphrase strictly matches Networks.TESTNET before simulating or signing transactions.
• Backend Indexer Validation: The suite actively polls the backend (/api/transactions/:hash) to confirm the emitted events/transactions are successfully indexed.
• Idempotency & Cleanup: Implemented a randomized 32-byte salt (runSalt) to generate unique, idempotent commitments per run. The test strictly concludes with a refund call to recover testnet XLM and clean up the contract state.
• Rate Limiting: Added synthetic delays between transactions to prevent spamming the Soroban RPC.

Acceptance Criteria

• CI can run E2E safely without leaking secrets or requiring private key custody in the app.
• Tests fail with actionable output (which step, which tx hash, which event missing).
• Running the suite multiple times produces deterministic results (handled via idempotent salts and proper cleanup).

How to Test

1. Ensure E2E_WALLET_SECRET and QUICKEX_CONTRACT_ID are configured in GitHub Actions Secrets.
2. Navigate to the Actions tab in GitHub.
3. Select Backend Soroban E2E (Testnet) and click Run workflow.
4. Verify the logs successfully deposit, validate the indexer, and process the refund cleanup without leaking any secret keys.

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[Cedarich]

@shadrach68 fix workflow

[Cedarich]

@shadrach68

[shadrach68]

Please, I am currently working on it, sir @Cedarich

[shadrach68]

I have resolved the issue sir @Cedarich