Code quality improvements: robust error handling, HA best practices, stale translation cleanup

[Copilot]

Complete code review pass addressing fragile error detection, HA anti-patterns, dead code, and stale/missing translation keys.

coordinator.py

• Replace fragile strerror string match with isinstance(exc, socket.gaierror) — the original check only caught one locale-specific DNS error message and missed others (e.g. "Name or service not known")
• Add _LOGGER.warning() in all exception handlers — _LOGGER was defined but never used; log messages now include host/username context for easier diagnosis
• Fix misleading default known-hosts path — Path('~', '.ssh', CONF_KNOWN_HOSTS) coincidentally worked (the constant value equals the filename) but was semantically wrong; replaced with the literal "known_hosts"

# Before — locale-sensitive, misses other gaierror variants
except OSError as e:
    if e.strerror == 'Temporary failure in name resolution':
        raise ServiceValidationError(...)

# After — catches all address-resolution failures
except OSError as exc:
    if isinstance(exc, socket.gaierror):
        _LOGGER.warning("Host %s is not reachable: %s", host, exc)
        raise ServiceValidationError(...)

config_flow.py

• Add single_instance_allowed = True — the HA-native class attribute for single-instance integrations; allows the framework to abort duplicate flows before async_step_user is reached (HA ≥ 2024.x)
• Remove hass.data.get(DOMAIN) redundant check — non-standard secondary guard that required # pylint: disable=no-member; _async_current_entries() is the correct HA pattern
• Remove unused logging / _LOGGER

strings.json / translations/en.json / translations/de.json

• Remove stale script_file service field and script_file_not_found exception (feature was replaced by input)
• Add missing password and input service fields
• Fix command_or_input message: "script file" → "input"
• Add missing integration_not_set_up exception (used in __init__.py but absent from all translation files)

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