refactor(backend): flatten leaf modules

Co-authored-by: Codex <noreply@openai.com>

backend/apps/gateway/tests/architecture.rs

@@ -383,7 +383,9 @@ fn workspace_modules_follow_conventional_file_layout() {
     let targets = test_source_roots();
     for member in WORKSPACE_MEMBERS {
         let member_root = backend_root().join(member);
-        assert_module_tree(&member_root.join("src"), &["lib.rs", "main.rs"]);
+        let src = member_root.join("src");
+        assert_module_tree(&src, &["lib.rs", "main.rs"]);
+        assert_directory_modules_have_children(&src, &["lib.rs", "main.rs"]);
 
         let tests = member_root.join("tests");
         if tests.is_dir() {
@@ -396,6 +398,29 @@ fn workspace_modules_follow_conventional_file_layout() {
     }
 }
 
+fn assert_directory_modules_have_children(root: &Path, crate_roots: &[&str]) {
+    let files = super::rust_files(root);
+    for relative in &files {
+        if crate_roots
+            .iter()
+            .any(|candidate| relative == Path::new(candidate))
+            || relative.file_name().and_then(|value| value.to_str()) != Some("mod.rs")
+        {
+            continue;
+        }
+
+        let directory = relative.parent().expect("mod.rs parent");
+        assert!(
+            files
+                .iter()
+                .any(|candidate| candidate != relative && candidate.starts_with(directory)),
+            "{} is a leaf module and must use {}",
+            root.join(relative).display(),
+            root.join(directory).with_extension("rs").display(),
+        );
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 enum ModuleLayout {
     File,

backend/crates/gateway-api/tests/architecture.rs

@@ -68,7 +68,7 @@ fn source_tree_should_match_frozen_machine_manifest() {
         "src/lib.rs",
         "src/middleware/headers.rs",
         "src/middleware/mod.rs",
-        "src/middleware/websocket/mod.rs",
+        "src/middleware/websocket.rs",
         "src/openai/middleware/http.rs",
         "src/openai/middleware/mod.rs",
         "src/openai/middleware/request.rs",
@@ -87,7 +87,7 @@ fn source_tree_should_match_frozen_machine_manifest() {
         "src/openai/responses/response.rs",
         "src/openai/responses/validation.rs",
         "src/openai/responses/websocket/connection/mod.rs",
-        "src/openai/responses/websocket/connection/middleware/mod.rs",
+        "src/openai/responses/websocket/connection/middleware.rs",
         "src/openai/responses/websocket/forward.rs",
         "src/openai/responses/websocket/mod.rs",
         "src/openai/responses/websocket/protocol.rs",
@@ -136,7 +136,7 @@ fn test_tree_should_match_frozen_rust_mirror() {
         "tests/key_usage/fixtures.rs",
         "tests/main.rs",
         "tests/middleware/mod.rs",
-        "tests/middleware/websocket/mod.rs",
+        "tests/middleware/websocket.rs",
         "tests/openai/middleware/mod.rs",
         "tests/openai/auth.rs",
         "tests/openai/error.rs",
@@ -148,7 +148,7 @@ fn test_tree_should_match_frozen_rust_mirror() {
         "tests/openai/responses/mod.rs",
         "tests/openai/responses/request.rs",
         "tests/openai/responses/websocket/connection/mod.rs",
-        "tests/openai/responses/websocket/connection/middleware/mod.rs",
+        "tests/openai/responses/websocket/connection/middleware.rs",
         "tests/openai/responses/websocket/forward.rs",
         "tests/openai/responses/websocket/interrupt.rs",
         "tests/openai/responses/websocket/mod.rs",