test: move application tests to tests/

test_support/test_helpers -> tests/support; watched_at unified w/ domain test_watched_at (2024-03-15).
Kept inline: users::diary_filter tests (feed_sort_to_direction_all_variants, paged_user_filter_builds_correctly) - pub(super) helpers in private module.
Dropped stale never-compiled users/tests/get_users.rs (old execute signature).

crates/application/tests/users/get_federated_profile_test.rs

@@ -0,0 +1,115 @@
+use std::sync::Arc;
+
+use async_trait::async_trait;
+use domain::{errors::DomainError, models::FederatedProfile, ports::FederatedProfileQuery};
+
+use application::users::deps::GetFederatedProfileDeps;
+use application::users::get_federated_profile;
+
+fn a_profile() -> FederatedProfile {
+    FederatedProfile {
+        actor_url: "https://remote.example/users/alice".into(),
+        handle: "alice@remote.example".into(),
+        display_name: Some("Alice".into()),
+        bio: None,
+        avatar_url: None,
+        banner_url: None,
+    }
+}
+
+struct FoundFederatedProfileQuery;
+
+#[async_trait]
+impl FederatedProfileQuery for FoundFederatedProfileQuery {
+    async fn get_federated_profile(
+        &self,
+        _synthetic_user_id: uuid::Uuid,
+    ) -> Result<Option<FederatedProfile>, DomainError> {
+        Ok(Some(a_profile()))
+    }
+}
+
+struct EmptyFederatedProfileQuery;
+
+#[async_trait]
+impl FederatedProfileQuery for EmptyFederatedProfileQuery {
+    async fn get_federated_profile(
+        &self,
+        _synthetic_user_id: uuid::Uuid,
+    ) -> Result<Option<FederatedProfile>, DomainError> {
+        Ok(None)
+    }
+}
+
+struct FailingFederatedProfileQuery;
+
+#[async_trait]
+impl FederatedProfileQuery for FailingFederatedProfileQuery {
+    async fn get_federated_profile(
+        &self,
+        _synthetic_user_id: uuid::Uuid,
+    ) -> Result<Option<FederatedProfile>, DomainError> {
+        Err(DomainError::InfrastructureError(
+            "remote lookup failed".into(),
+        ))
+    }
+}
+
+#[tokio::test]
+async fn returns_the_profile_when_the_port_finds_one() {
+    let deps = GetFederatedProfileDeps {
+        federated_profile: Some(Arc::new(FoundFederatedProfileQuery)),
+    };
+
+    let result = get_federated_profile::execute(&deps, uuid::Uuid::new_v4())
+        .await
+        .unwrap();
+
+    assert_eq!(
+        result.map(|p| p.handle),
+        Some("alice@remote.example".to_string())
+    );
+}
+
+#[tokio::test]
+async fn returns_ok_none_when_the_port_finds_nothing() {
+    let deps = GetFederatedProfileDeps {
+        federated_profile: Some(Arc::new(EmptyFederatedProfileQuery)),
+    };
+
+    let result = get_federated_profile::execute(&deps, uuid::Uuid::new_v4())
+        .await
+        .unwrap();
+
+    assert!(result.is_none());
+}
+
+#[tokio::test]
+async fn returns_ok_none_when_federation_is_disabled() {
+    let deps = GetFederatedProfileDeps {
+        federated_profile: None,
+    };
+
+    let result = get_federated_profile::execute(&deps, uuid::Uuid::new_v4())
+        .await
+        .unwrap();
+
+    assert!(
+        result.is_none(),
+        "an absent port must collapse to Ok(None), the same as an empty lookup — \
+         the handler cannot and must not tell the two apart"
+    );
+}
+
+#[tokio::test]
+async fn propagates_an_error_from_the_port_unchanged() {
+    let deps = GetFederatedProfileDeps {
+        federated_profile: Some(Arc::new(FailingFederatedProfileQuery)),
+    };
+
+    let err = get_federated_profile::execute(&deps, uuid::Uuid::new_v4())
+        .await
+        .expect_err("a repository error must not be swallowed inside the use case");
+
+    assert!(matches!(err, DomainError::InfrastructureError(_)));
+}