`^C` should not unconditionally terminate the session.

Instead of sending `SIGINT` to the shell and terminating the current process (and providing a new prompt), `^C` unconditionally terminates the SSM session, even if `su - ` has been used, or if a process is in the foreground `tail -f /path/to/file`), or from within a nested shell. This behavior is unique to SSM sessions, so I imagine the agent is listening for `SIGINT` and terminating.... which is fairly cumbersome.

```
Starting session with SessionId: REDACTED
exec bash
sh-4.2$ exec bash
[ssm-user@ip-123 bin]$ stty -a
speed 38400 baud; rows 61; columns 240; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = ; eol2 = ; swtch = ; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd -cmspar cs8 -hupcl -cstopb cread -clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff -iuclc -ixany -imaxbel -iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke
[ssm-user@ip-123 bin]$ stty intr \^C
[ssm-user@ip-123 bin]$ ^C
[ssm-user@ip-123 bin]$

$ # now i'm back in my local prompt
```

Reproduce the SSM session behavior shown in the report, including nested shells, su, and a foreground tail -f process, then trace how the agent handles ^C and SIGINT. Done means ^C interrupts the foreground command or shell without unconditionally terminating the SSM session or returning to the local prompt.

Written by the indexing model from the issue text.