Notebook markdown renderer emits duplicate heading ids

### Bug description

The notebook markdown renderer's heading id generation (`extensions/markdown-language-features/notebook/index.ts`, `addNamedHeaderRendering`) deduplicates slugs with a counter, but never registers the generated `-N` suffixed id it emits. Only the base slug is recorded.

For headings `Foo`, `Foo-1`, `Foo`:

- expected (github-slugger semantics): `foo`, `foo-1`, `foo-2`
- observed: `foo`, `foo-1`, `foo-1` because the third heading builds its deduped id from the base slug counter and emits `foo-1` again without noticing that id was already handed out earlier.

Duplicate DOM ids mean anchor links from github.com do not scroll to the right heading inside the notebook webview.

### Steps to reproduce

1. Render notebook markdown containing headings `Foo`, `Foo-1`, `Foo`.
2. Inspect the rendered header ids: the second and third both get `foo-1`.

### Expected behavior

Every emitted id, including generated `-N` suffixes, is registered so later duplicates advance until a free id, matching github-slugger output (`foo-2` for the third heading).

### Version tested

Commit `38ec3d57f91b` on `main`; reproduced by running the renderer module through the real markdown-it pipeline and inspecting produced ids. A fix is ready.