fix: don't show file tree view in playground output for single file (#10212)

When an emitter produces a single output file nested in a folder (e.g.
`src/openapi.yaml`), the playground unnecessarily renders the full tree
view with folder navigation panel. This adds clutter for emitters whose
output is always nested.

- Updated `showFileTree` condition in `file-viewer.tsx` to require
`outputFiles.length > 1` before considering folder nesting:

```tsx
// Before
outputFiles.some((f) => f.includes("/")) || outputFiles.length >= 3

// After
outputFiles.length > 1 && (outputFiles.some((f) => f.includes("/")) || outputFiles.length >= 3)
```

Single nested file now renders with the simple tab view; tree view still
activates for 2+ nested files or 3+ flat files.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: timotheeguerin <1031227+timotheeguerin@users.noreply.github.com>

Author: Copilot

.chronus/changes/fix-playground-single-file-tree-view-2026-3-31-14-1-33.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/playground"
+---
+
+Don't show file tree view in output when there is only a single file

packages/playground/src/react/output-view/file-viewer.tsx

@@ -18,7 +18,9 @@ const FileViewerComponent = ({
   const [content, setContent] = useState<string>("");
 
   const showFileTree = useMemo(
-    () => outputFiles.some((f) => f.includes("/")) || outputFiles.length >= 3,
+    () =>
+      outputFiles.length > 1 &&
+      (outputFiles.some((f) => f.includes("/")) || outputFiles.length >= 3),
     [outputFiles],
   );