Fix preview of single-file Julia documents using Jupyter instead of Julia engine

Engine extensions (like the bundled Julia engine) were only loaded in
singleFileProjectContext when renderOptions was provided. The preview
command creates the project context before render services exist, so
engine extensions were never discovered for single files. This caused
Jupyter to win the language claim for Julia code blocks.

Now engine extensions are always resolved, using a fresh extension
context as fallback when renderOptions is absent — matching the pattern
used by projectContext.

Fixes #14208

Author: gordonwoodhull

src/project/types/single-file/single-file.ts

@@ -35,6 +35,7 @@ import {
   mergeExtensionMetadata,
   resolveEngineExtensions,
 } from "../../project-context.ts";
+import { createExtensionContext } from "../../../extension/extension.ts";
 
 export async function singleFileProjectContext(
   source: string,
@@ -93,17 +94,19 @@ export async function singleFileProjectContext(
       result.diskCache.close();
     },
   };
+  // Always resolve engine extensions so bundled engines (e.g. Julia) are
+  // discovered even when called without renderOptions (e.g. from preview)
+  const extensionContext = renderOptions?.services.extension ||
+    createExtensionContext();
+  result.config = result.config || { project: {} };
+  result.config = await resolveEngineExtensions(
+    extensionContext,
+    result.config,
+    result.dir,
+  );
+
   if (renderOptions) {
-    result.config = {
-      project: {},
-    };
-    // First resolve engine extensions
-    result.config = await resolveEngineExtensions(
-      renderOptions.services.extension,
-      result.config,
-      result.dir,
-    );
-    // Then merge extension metadata
+    // Merge extension metadata (requires renderOptions for full services)
     await mergeExtensionMetadata(result, renderOptions);
 
     // Check if extensions contributed output-dir metadata