test(docs): fix test failure due to bad merge

Fixes a semantic merge conflict due to the PRs:
  - https://github.com/mermaid-js/mermaid/pull/3954
    Changed `docs.mts` to use a remark object created by `remark()`
  - https://github.com/mermaid-js/mermaid/pull/3946
    Added test code that mocked the frozen remark object
    (e.g. `remark` not `remark()`).

To fix this issue, we can mock `remark()` so that it always returns
the same remark object, which can then be used the `docs.mts` script,
as well as spied on in the `docs.spec.ts` test file.

Reported-by: Sidharth Vinod <sidharthv96@gmail.com>

packages/mermaid/src/docs.spec.ts

@@ -1,7 +1,21 @@
 import { transformBlocks, transformToBlockQuote } from './docs.mjs';
 
-import { remark } from 'remark'; // import it this way so we can mock it
-vi.mock('remark');
+import { remark as remarkBuilder } from 'remark'; // import it this way so we can mock it
+
+const remark = remarkBuilder();
+
+vi.mock('remark', async (importOriginal) => {
+  const { remark: originalRemarkBuilder } = (await importOriginal()) as {
+    remark: typeof remarkBuilder;
+  };
+
+  // make sure that both `docs.mts` and this test file are using the same remark
+  // object so that we can mock it
+  const sharedRemark = originalRemarkBuilder();
+  return {
+    remark: () => sharedRemark,
+  };
+});
 
 afterEach(() => {
   vi.restoreAllMocks();