Fix ${workspaceFolder} resolution and relative path handling for defaultInterpreterPath

[Copilot]

python.defaultInterpreterPath failed to resolve in two cases: ${workspaceFolder} was left unexpanded when VS Code couldn't determine the owning folder (e.g. Windows drive-letter casing), and relative paths were resolved against an unrelated CWD by the native finder—producing malformed, duplicated paths like <workspace>/<workspace-name>/.venv/....

Changes

• internalVariables.ts — When getWorkspaceFolder() returns undefined for a project scope, fall back to the single open workspace folder so ${workspaceFolder} still expands.
• interpreterSelection.ts — Added toAbsoluteInterpreterPath() to resolve relative interpreter paths against the workspace folder (with the same single-folder fallback) before handing them to the native finder. Absolute paths pass through unchanged.
• Tests — Added unit coverage for the new edge cases:
  • ${workspaceFolder} single-folder fallback, multi-folder no-op, no-scope, and no-folders-open cases.
  • Relative path resolution via owning folder and via single-folder fallback, multi-folder pass-through, and absolute-path pass-through.

Notes

• resolveVariables() is separator-preserving string substitution, so the new assertions intentionally compare forward-slash inputs rather than path.join(). Relative-path expectations use path.resolve() / Uri.file().fsPath to stay cross-platform.