Fix call hierarchy file node paths

[Copilot]

TypeScript 7 call hierarchy file nodes displayed absolute paths, making large hierarchies difficult to scan.

• Display: Use the file basename as the node name.
• Context: Show the project-relative containing directory as detail.
• Navigation: Preserve the absolute file URI.
• Coverage: Add a subdirectory regression case and update affected call hierarchy baselines.

Before: /workspace/src/callers.ts
After:  callers.ts  src

• Fixes On TypeScript 7, Call hierarchy shows a full absolute path instead of file name + relative path #64097

[Jake Bailey (jakebailey)]

I'm not sure this is 100% right; it turns out that the old VS Code extension did vscode.workspace.asRelativePath(path.dirname(item.file)) which actually made the path relative to the workspace root.

I think we'd have to somehow plumb that info down from the init message.