This PR adds handling for the case where the user's gemini-cli binary
fails to start up because it's too old to support the
`--experimental-acp` flag. We previously had such handling, but it got
lost as part of #35578.

This doesn't yet handle the case where the server binary exits
unexpectedly after the connection is established; that'll be dealt with
in a follow-up PR since it needs different handling and isn't specific
to gemini-cli.

Release Notes:

- N/A

Co-authored-by: Agus <agus@zed.dev>