Add unit test for single-file engine extension resolution without renderOptions

Verifies that singleFileProjectContext discovers bundled engine
extensions (e.g. Julia) even when called without renderOptions,
which is the code path used by quarto preview.

Co-authored-by: Christophe Dervieux <[email protected]>

Author: gordonwoodhull

tests/unit/project/single-file-engine-resolution.test.ts

@@ -0,0 +1,47 @@
+/*
+ * single-file-engine-resolution.test.ts
+ *
+ * Tests that singleFileProjectContext resolves engine extensions
+ * even without renderOptions (the preview code path).
+ * Related to issue #13983
+ *
+ * Copyright (C) 2026 Posit Software, PBC
+ */
+
+import { unitTest } from "../../test.ts";
+import { assert } from "testing/asserts";
+import { join } from "../../../src/deno_ral/path.ts";
+import { singleFileProjectContext } from "../../../src/project/types/single-file/single-file.ts";
+import { notebookContext } from "../../../src/render/notebook/notebook-context.ts";
+import { initYamlIntelligenceResourcesFromFilesystem } from "../../../src/core/schema/utils.ts";
+
+unitTest(
+  "singleFileProjectContext resolves engine extensions without renderOptions",
+  async () => {
+    await initYamlIntelligenceResourcesFromFilesystem();
+
+    const tmpDir = Deno.makeTempDirSync({ prefix: "quarto-test" });
+    const file = join(tmpDir, "test.qmd");
+    Deno.writeTextFileSync(
+      file,
+      "---\ntitle: test\nengine: julia\n---\n",
+    );
+
+    try {
+      const nbContext = notebookContext();
+      const project = await singleFileProjectContext(file, nbContext);
+
+      assert(
+        project.config !== undefined,
+        "config should be initialized even without renderOptions",
+      );
+      assert(
+        Array.isArray(project.config?.engines) &&
+          project.config.engines.length > 0,
+        "engine extensions should be resolved (bundled engines discovered)",
+      );
+    } finally {
+      Deno.removeSync(tmpDir, { recursive: true });
+    }
+  },
+);