Check that input is a tty before using it

[afbjorklund]

Closes #4410

Also makes sure that shell checks the --tty flag

Could probably use some tests, this is code only

[afbjorklund]

Added a test, here are the results with the old code:

 ✗ lima stopped lima instance
   (in test file hack/bats/tests/shell.bats, line 20)
     `limactl shell --tty=false "$NAME" true </dev/null' failed
? Do you want to start the instance now?  (Y/n) time="2025-12-17T07:53:09+01:00" level=fatal msg=EOF
 ✗ yes | stopped lima instance
   (in test file hack/bats/tests/shell.bats, line 25)
     `bash -c "yes | limactl shell --tty=true $NAME true"' failed with status 2
? Do you want to start the instance now?  (Y/n) panic: runtime error: invalid memory address or nil pointer dereference
   [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xfa56f6]

Now the results are consistent, although confusing:

• The --yes option is inconsistent and confusing #3995

i.e. the yes is discarded (like --yes).

But at least it is not crashing anymore...