Feature/issue 738 proxy test

[Immaculate0606]

Description:

Closes #738

Description

This PR upgrades the tests/integration/proxy.test.ts suite to utilize a real PostgreSQL database via testcontainers, enabling end-to-end testing of the proxy's idempotencyMiddleware which previously skipped its assertions when a database connection was unavailable.

Changes

• Testcontainers Integration: Added GenericContainer('postgres:16-alpine') configuration within the beforeAll block to spin up a fully isolated PostgreSQL container on a dynamic port during testing.
• Database Schema & Migrations: Added a runAllMigrations helper to dynamically apply all .sql schema definitions from the migrations/ directory to the test container before any requests are handled.
• Dependency Injection: Updated the buildProxyApp test harness to accept the actual Pool instance from the testcontainers context and properly bind it to app.locals.dbPool, ensuring the idempotency layer hooks into the container instead of failing.
• Idempotency Coverage: Removed the PG may be unavailable bypass in the idempotency POST test. It now strictly validates that Idempotency-Key correctly deduplicates matching requests and serves the cached response accurately.

Testing Instructions

1. Run npm test tests/integration/proxy.test.ts locally with a running Docker daemon.
2. Observe that the "Idempotency test skipped" warning is gone, and the tests execute correctly against the temporary database container.

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[greatest0fallt1me]

Nice work, everything checks out. Merged 🎉