fix(course-format): directories are not lesson files; a failing course never rejects loadAll

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/course-format/test/load.test.ts

@@ -80,6 +80,34 @@ describe("overlays and errors", () => {
     expect(errs[2]!.message).toContain("Exercise 1");
   });
 
+  test("lesson file that is a directory gives a diagnostic", async () => {
+    const d = await tmp();
+    await Bun.write(join(d, "adir/x.md"), "# X\n");
+    await Bun.write(join(d, "stcourse.yaml"), manifest("dirs", "", "      - { id: as-file, title: L, type: text, file: adir }"));
+    const c = await loadCourse(join(d, "stcourse.yaml"));
+    expect(c.status).toBe("error");
+    const e = c.diagnostics.filter((x) => x.level === "error");
+    expect(e).toHaveLength(1);
+    expect(e[0]!.lessonId).toBe("as-file");
+    expect(e[0]!.path).toBe("adir");
+  });
+
+  test("one throwing course does not break loadAll", async () => {
+    const d = await tmp();
+    await Bun.write(join(d, "bad/adir/x.md"), "# X\n");
+    await Bun.write(join(d, "bad/a.md"), "# A\n## Sec\n");
+    await Bun.write(
+      join(d, "bad/stcourse.yaml"),
+      manifest("bad", "", "      - { id: h, title: L, type: exercise, file: a.md, hints: { file: adir, match: 'E' } }"),
+    );
+    await Bun.write(join(d, "good/a.md"), "# A\n");
+    await Bun.write(join(d, "good/stcourse.yaml"), manifest("good", "", "      - { id: l, title: L, type: text, file: a.md }"));
+    const all = await loadAll([d]);
+    expect(all.map((c) => c.id).sort()).toEqual(["bad", "good"]);
+    expect(all.find((c) => c.id === "bad")!.status).toBe("error");
+    expect(all.find((c) => c.id === "good")!.status).toBe("ok");
+  });
+
   test("invalid manifest gives an error course and does not throw", async () => {
     const d = await tmp();
     await Bun.write(join(d, "stcourse.yaml"), "schema: 2\nid: x\n");