fix(html): preserve ? on optional tuple elements

[fibibot]

Summary

Reported in denoland/deno#29669. `render_type_def` in `src/html/types.rs` handled `TsTypeDefKind::Optional` (the kind produced for the trailing-`?` form in tuple element types like `[T, U?]`) by returning the inner type's HTML and dropping the `?`:

```rust
TsTypeDefKind::Optional(inner) => render_type_def(ctx, inner),
```

So this source:

```ts
export function a(x: [number, number?]): number {
return x[0] + (x?.[1] ?? 0);
}
```

rendered as `[number, number]` in HTML even though terminal/JSON output (which goes through `Display` and emits `{ty}?`) was correct. Mirror the `Display` behaviour for HTML.

Test plan

• Existing snapshot tests `html_test__symbol_group.snap` and the `html_doc_files_multiple` family already exercise an `optionalMethod(): [string?]` symbol; they were silently capturing the buggy output. Snapshots refreshed under `INSTA_UPDATE=always` and now show the `?` (e.g. `[<span class="td-kw">string?]`).
• Manual repro from the issue: `deno doc --html` on `export function a(x: [number, number?]): number` now renders `[number, number?]` in the parameters table.
• `cargo fmt --check`, `cargo clippy --tests -- -D warnings`.

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