Closes #182

[p3ris0n]

Closes #182

Description
This PR implements a robust, production-ready API Test Client within backend/src/test_utils/client.rs. Designed specifically for our Axum, SQLx (PostgreSQL), and Redis stack, this test client simplifies integration testing by mocking HTTP requests, handling session/auth states, and ensuring clean database transactions across test suites.
Changes Made
Core Test Client (backend/src/test_utils/client.rs):
Implemented an isolated TestClient struct that wraps our Axum router using tower::ServiceExt.
Added helper methods for common HTTP verbs (get, post, put, delete).
Integrated automatic header injection (e.g., Bearer tokens for authenticated routes).
State & Dependency Mocking:
Added setups for isolated PostgreSQL transactions that roll back automatically after each test run using SQLx.
Integrated a localized Redis pool/mock client connection handling to prevent test state pollution.
Observability:
Configured tracing-subscriber initialization within the test environment for easier debugging during test failures.
Documentation:
Updated backend/README.md with clear examples of how to spin up and use the new TestClient in integration tests.
Testing Strategy
Integration Tests: Added full-suite validation ensuring the TestClient correctly hits Axum endpoints, processes data via SQLx, and interacts with Redis without leaks.
Run the suite using:
Bash
cd backend
cargo test --lib test_utils::client
Checklist
[x] Feature is fully implemented and verified locally.
[x] Code compiles cleanly without any clippy or compiler warnings.
[x] All unit and integration tests pass successfully.
[x] Documentation added/updated in backend/README.md and inline via Rustdoc.
[x] No breaking changes introduced to existing APIs.
Example Usage (Added to README)
Rust
#[tokio::test]
async fn test_example_endpoint() {
// Initialize test database, redis, and app state
let client = TestClient::new().await;

// Send a mock POST request
let response = client.post("/api/v1/resource")
    .json(&json!({ "name": "Crucible Test" }))
    .send()
    .await;

assert_eq!(response.status(), StatusCode::CREATED);

}

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