feat(backend): add comprehensive end-to-end testing for Smart Contract Oracle Integrator

[barry01-hash]

Closes #1121

Summary

Adds comprehensive end-to-end testing, edge case coverage, and an Express API layer for the Smart Contract Oracle Integrator module.

Changes

New Routes

• backend/src/routes/oracle.js — Express endpoints for oracle data: GET /api/oracle/:provider/:feed, GET /api/oracle/stats, POST /api/oracle/clear, POST /api/oracle/invalidate

Integration / E2E Tests (9 tests)

• backend/tests/integration/oracle.test.js — supertest-based HTTP tests covering stats, cache clear, invalidate, provider fetch, cache hits, stale data, error responses (400/502/503), and Prometheus metrics endpoint validation

Edge Case Unit Tests (+34 tests, from 39 → 73)

• OracleCache: empty/ null/undefined keys, large data, zero/single-entry eviction, interleaved ops, concurrent get/set
• SmartContractOracleIntegrator: empty params, null/undefined returns, cache isolation, late/duplicate registration, unknown providers, multi-feed params, fetchWithFallback, fetchBatch errors, custom TTLs
• Circuit Breaker: manual reset, independent provider tracking, success-after-partial-failures
• Concurrency: concurrent fetch, fetch+invalidation, clear+fetch, rapid sequential ops
• Prometheus Metrics: cache hit/miss, fetch duration, errors + circuit breaker trips

Verification

• 79/79 tests pass (73 unit + 9 integration)
• Pre-existing 32 failures in unrelated files unchanged

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