Leverage execution.exitCode for execution promise resolve

https://github.com/microsoft/vscode-python-environments/pull/160#discussion_r1946083357

We have

                    let timer: NodeJS.Timeout | undefined = setTimeout(() => {
                        execPromise.resolve();
                        traceError(`Shell execution timed out: ${command.executable} ${command.args?.join(' ')}`);
                    }, 2000);

in src/features/terminal/terminalActivationState.ts

We could do a Promise race where we resolve faster, via listening to .onDidEndTerminalShellExecution and watching if exitCode returned with values other than undefined.

Also we should watch for exitCode regardless to ensure if command failed/succeeded. (For example, in the middle of the activation, what if user interrupted and stopped? We should know this)