feat: add comprehensive Rust API tests with positive and negative cases (#27)

Implement 44 comprehensive unit tests for the Rust Actix-web API following
industry best practices for API testing:

Test Coverage:
- Root endpoint tests (4 tests)
- Health endpoint tests (11 tests: positive + negative)
- Vault endpoint tests (5 tests: positive + negative)
- Cache endpoint tests (13 tests: GET/POST/DELETE + edge cases)
- Messaging endpoint tests (3 tests)
- Redis cluster endpoint tests (6 tests)
- Metrics endpoint tests (3 tests)
- Edge cases and error handling (9 tests)

Key Features:
- Positive test cases: Valid inputs, expected success responses
- Negative test cases: Invalid inputs, error conditions, edge cases
- HTTP status code validation (200, 404, 405, 400, 503)
- JSON response structure validation
- Empty/missing parameter handling
- Special character handling
- Very long input handling
- Zero/negative TTL handling

Test Organization:
- Grouped by endpoint/feature for maintainability
- Clear test names describing what is being tested
- Comprehensive documentation with comments
- Macro-based test app creation for DRY code

All 44 tests pass successfully with proper error handling
and validation of both success and failure scenarios.

Author: NormB

reference-apps/rust/src/main.rs

@@ -1340,45 +1340,4 @@ async fn main() -> std::io::Result<()> {
 }
 
 #[cfg(test)]
-mod tests {
-    use super::*;
-    use actix_web::{test, App};
-
-    #[actix_web::test]
-    async fn test_root_endpoint() {
-        let app = test::init_service(App::new().route("/", web::get().to(root))).await;
-        let req = test::TestRequest::get().uri("/").to_request();
-        let resp = test::call_service(&app, req).await;
-
-        assert!(resp.status().is_success());
-
-        let body: ApiInfo = test::read_body_json(resp).await;
-        assert_eq!(body.name, "DevStack Core Reference API");
-        assert_eq!(body.version, "1.0.0");
-        assert_eq!(body.language, "Rust");
-        assert_eq!(body.framework, "Actix-web");
-    }
-
-    #[actix_web::test]
-    async fn test_health_endpoint() {
-        let app = test::init_service(App::new().route("/health/", web::get().to(health_simple))).await;
-        let req = test::TestRequest::get().uri("/health/").to_request();
-        let resp = test::call_service(&app, req).await;
-
-        assert!(resp.status().is_success());
-
-        let body: HealthResponse = test::read_body_json(resp).await;
-        assert_eq!(body.status, "healthy");
-        assert!(body.timestamp.is_some());
-    }
-
-    #[actix_web::test]
-    async fn test_metrics_endpoint() {
-        register_metrics();
-        let app = test::init_service(App::new().route("/metrics", web::get().to(metrics))).await;
-        let req = test::TestRequest::get().uri("/metrics").to_request();
-        let resp = test::call_service(&app, req).await;
-
-        assert!(resp.status().is_success());
-    }
-}
+mod tests;  // Comprehensive test suite in tests.rs