feat: add sanitization check, refactor error returns

tests/health_check.rs

@@ -70,7 +70,7 @@ async fn subscribe_returns_a_422_when_data_is_missing() {
         assert_eq!(
             422,
             response.status().as_u16(),
-            "The API did not fail with 400 Bad Request when the payload was {}.",
+            "The API did not fail with 422 when the payload was {}.",
             error_message
         );
     }
@@ -104,6 +104,37 @@ async fn subscribe_returns_a_200_for_valid_form_data() {
     assert_eq!(saved.name, "le guin");
 }
 
+#[tokio::test]
+async fn subscribe_returns_a_200_when_fields_are_present_but_empty() {
+    // Arrange
+    let app = spawn_app().await;
+    let client = reqwest::Client::new();
+    let test_cases = vec![
+        ("name=&email=ursula_le_guin%40gmail.com", "empty name"),
+        ("name=Ursula&email=", "empty email"),
+        ("name=Ursula&email=definitely-not-an-email", "invalid email"),
+    ];
+
+    for (body, description) in test_cases {
+        // Act
+        let response = client
+            .post(&format!("{}/subscriptions", &app.address))
+            .header("Content-Type", "application/x-www-form-urlencoded")
+            .body(body)
+            .send()
+            .await
+            .expect("Failed to execute request.");
+
+        // Assert
+        assert_eq!(
+            200,
+            response.status().as_u16(),
+            "The API did not return a 200 OK when the payload was {}.",
+            description
+        );
+    }
+}
+
 async fn spawn_app() -> TestApp {
     // The first time `initialize` is invoked the code in `TRACING` is executed.
     // All other invocations will instead skip execution.