[v3-3-test] Stop Breeze container SSH setup from modifying the host ~/.ssh (#72182)

[github-actions]

When Breeze runs with --forward-credentials, the host's ~/.ssh is
bind-mounted read-write at /root/.ssh and the container start used to
operate on it directly. Every start overwrote the user's id_rsa with a
throwaway key, appended that key to authorized_keys and three localhost
entries to known_hosts (observed to accumulate hundreds of entries), left
a dangling authorized_keys2 symlink, and ran 'chmod 600 ~/.ssh/*'. That
chmod also strips the execute bit from the ~/.ssh/agent directory that
OpenSSH 10.x uses for agent sockets. On macOS this kills the
launchd-managed ssh-agent and, after the next reboot, makes every ssh on
the host hang waiting on the dead agent socket.

The ssh-to-localhost setup, which the real-connection SSH and SFTP
provider tests rely on, now lives entirely in /root/.breeze-ssh, a path
that is never mounted from the host. sshd accepts the generated key via
an sshd_config.d drop-in, the OpenSSH client finds it via an ssh_config.d
drop-in, and paramiko-based hooks, which read neither, discover it
through an ssh-agent started by the entrypoint. entrypoint_exec.sh
exports the agent socket so 'breeze exec' shells get it too. The
authorized_keys2 symlink had no consumers anywhere in the repo and is
dropped.
(cherry picked from commit 51ebe32)

Co-authored-by: Ramit Kataria hi@ramit.ca