Function createMockUri

Creates the URI namespace subset used by this kit (parse, file, joinPath). Returned values preserve scheme/path/toString facts needed by adapters; they do not implement authority, query, fragment, with, JSON serialization or platform-specific fsPath normalization.

VS Code's URI is nominal, while this fixture returns structural plain data. Tests should assert on the documented fields/string conversion, not instanceof or class identity. Cast only at the fixture boundary when a function signature requires the nominal vscode.Uri type.