Honor selected environment when creating Python terminals

[Copilot]

“Create Python Terminal” could ignore the clicked environment, do nothing without a workspace, or activate the project environment instead.

• Terminal creation

  • Prioritize the clicked environment.
  • Use project selection only for cwd.
  • Preserve multi-root cancellation behavior.

• Activation

  • Carry the requested environment through shell-startup fallback instead of re-resolving it from cwd.

• Regression coverage

  • Cover no workspace, unset/different project environments, picker cancellation, and shell fallback.

const cwd = project ? await findParentIfFile(project.uri.fsPath) : undefined;
await terminalManager.create(clickedEnvironment, { cwd });

• Fixes "Create Python Terminal" button doesn't work as expected #1549