Handle KeyImportError for invalid SSH private key files

[Copilot]

When a key file exists but contains invalid private key data, asyncssh raises KeyImportError, which was previously uncaught and surfaced as a generic "Unknown Error" in the UI.

Changes

• coordinator.py — Catch asyncssh.KeyImportError and raise a ServiceValidationError with translation key invalid_key_file
• strings.json, translations/en.json, translations/de.json — Add invalid_key_file exception message
• test_async_execute.py — Add test_invalid_key_file covering the new error path

except KeyImportError as exc:
    _LOGGER.warning("Invalid key file for %s@%s: %s", username, host, exc)
    raise ServiceValidationError(
        "The key file is not a valid private key.",
        translation_domain=DOMAIN,
        translation_key="invalid_key_file",
    ) from exc

Original prompt

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This section details on the original issue you should resolve

<issue_title>Unknown Error when key file exists but is not a valid private key</issue_title>
<issue_description>SSH Command version
v1.0.0

Home Assistant version
2026.3.2

Issue / Experienced behavior
"Unknown Error" is shown in the UI when the stated key file exists but is not a valid private key. The error in the logs is "asyncssh.public_key.KeyImportError: Invalid private key"

Expected behavior
A meaningful error should be presented to the user.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Unknown Error when key file exists but is not a valid private key #12

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[gensyn]

@copilot Please fix the unit tests that broke in this branch.

[Copilot]

@copilot Please fix the unit tests that broke in this branch.

Fixed in 22baf84. The test_invalid_key_file test had a stale patch("ssh_command.coordinator.exists") left over from before the coordinator was updated to use pathlib.Path.exists. Since SERVICE_DATA_BASE has no input and check_known_hosts=False, no exists call happens at all — so the patch was simply removed.