feat(stellar): add Horizon mock server and fix pool sequence resync & tests.

[ShantelPeters]

🚀 Summary of Changes

This PR introduces an in-memory Horizon Mock Server with configurable chaos/outage controls, adds end-to-end integration tests for Horizon node failover & outage recovery, fixes sequence resynchronization in the Stellar Channel Account Pool, and resolves test suite suite dependencies across SEP-12 KYC & trustline modules.

🛠️ Key Improvements & Features

1. 🌐 Mock Horizon Server (src/mocks/horizonMockServer.ts & scripts/horizon-mock-server.ts)

• Full Horizon API Simulation: Implements Express handlers for core Horizon REST endpoints (/, /health, /accounts/:id, /accounts/:id/transactions, /transactions, /fee_stats, /paths/strict-receive, /ledgers).
• Chaos Engineering & Outage Controls: Supports dynamic outage modes (503_service_unavailable, 500_internal_error, 429_rate_limit, timeout, flaky, offline), latency injection (delayMs), and Retry-After header simulations.
• CLI Command: Added npm run horizon-mock:dev to run the standalone mock server locally for development.

2. 🛡️ Network Outage & Failover Integration (src/stellar/__tests__/horizonOutage.test.ts)

• Added integration test coverage verifying HorizonPool automatic failover across nodes during 503 Service Unavailable, 500 Internal Error, and 429 Rate Limit conditions.
• Updated HorizonPool (src/stellar/horizonPool.ts) to allow HTTP URLs when running tests or in development (allowHttp).

3. 🔄 Channel Account Pool Sequence Resync Fix (src/stellar/pool.ts)

• Fixed submitWithChannel to update local account.sequence = newSeq upon detecting sequence mismatches (tx_bad_seq), ensuring retry attempts use the updated sequence number.
• Removed redundant duplicate sequence error checks in submitTransaction.

4. 🔑 SEP-12 Rate Limiting & UserModel Integration (src/middleware/rateLimit.ts & src/stellar/sep12.ts)

• Updated sep12RateLimiter key resolution to fallback to query/body account parameters or client IP when requests are unauthenticated.
• Updated Express route handlers in sep12.ts to preserve AppError.statusCode for proper HTTP 400 validation error responses.
• Mocked UserModel in sep12.test.ts to prevent database lockups during unit testing.

5. 🧪 Unit Test Normalization (src/stellar/__tests__/)

• Fixed StellarSdk.Account mock instantiations in trustlines.test.ts and payments.test.ts.
• Updated limit format expectations for ChangeTrust operations (1000.0000000, 0.0000000) matching StellarSdk 7-decimal string normalization.

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🧪 Verification & Testing

Automated Test Results

• All 12 Stellar Test Suites: 186 / 186 tests passed (100%)
• Horizon Mock Tests: 15 / 15 tests passed (100%)
• TypeScript Build: npm run build (tsc) compiled cleanly with 0 errors.

# Run Horizon Mock tests
npx jest src/mocks/tests/horizonMockServer.test.ts
# Run all Stellar test suites
npx jest src/stellar/__tests__/
# Build project
npm run build


CLOSES #1835 

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