Handle when onDidEndTerminalShellExecution never fires

[anthonykim1]

Related: #24078
We should rather have exitCode as a promise instead of the entire object as a promise for case:

• onDidEndTerminalShellExecution never fires because because the python command never finishes.
  @ The case of where shell integration is enabled in zsh/pwsh in Windows but not inside Python REPL so python command won't finish until user exit()

Cases we need to handle:

@ Shell integration in zsh/pwsh works, but not python for windows

1. executeCommand never actually runs the command because the python command never finishes
2. onDidEndTerminalShellExecution never fires because because the python command never finishes

This PR takes care of the second case scenario.

[anthonykim1]

I've tested this on windows and it does not seem like we get stuck in a infinite loop, regardless of this PR.

I'm not sure if this is because

if (terminal.shellIntegration) {

is smart enough to realize subshell (python shell inside pwsh for example), does not support shell integration (for windows python in terminal in our scenario)

OR

because we aren't awaiting this exitCode hence we are never stuck in the infinite loop, since executeCommand will just return even when it is not sending any command to terminal.

#24138 should help what is happening on windows in the pwsh => python scenario.