Fix SSH provider imports when sshtunnel is broken on Python 3.14 (#64258)

[deepujain]

Summary

The SSH provider imported sshtunnel at module import time, so Python 3.14 environments with the current incompatible sshtunnel release could fail on plain imports like SFTPHook before any tunnel functionality was used. This change defers the sshtunnel import until get_tunnel() is called and raises a clear optional-feature error if the dependency is unavailable or syntactically incompatible.

Changes

• providers/ssh/src/airflow/providers/ssh/hooks/ssh.py -- lazy-load sshtunnel only when tunnel creation is requested and raise AirflowOptionalProviderFeatureException on import/runtime incompatibility.
• providers/ssh/tests/unit/ssh/hooks/test_ssh.py -- add regression coverage for importing SSHHook when sshtunnel is broken and for the runtime error path when tunnel support is used.

Test plan

• uv run --project providers/sftp python - <<'PY' smoke import of from airflow.providers.sftp.hooks.sftp import SFTPHook
• uv run --project providers/sftp pytest providers/ssh/tests/unit/ssh/hooks/test_ssh.py -k 'test_tunnel_with_password or test_tunnel_without_password or test_tunnel_with_private_key or test_tunnel_with_private_key_passphrase or test_tunnel_with_private_key_ecdsa or test_can_import_ssh_hook_when_sshtunnel_is_broken or test_tunnel_requires_working_sshtunnel' -xvs
• CI passes (ruff, mypy, pytest)

Fixes #64258

[eladkal]

I don't think this is the right fix.
The idea is to make the provider compatible with Python 3.14 not just hide an import that does not work.

[eladkal]

Closing in favor of #64299