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)