Raise error when both password and key_file are provided

[Copilot]

Providing both password and key_file silently succeeded using the password, with no indication that the conflicting inputs were ignored. This adds an explicit validation error when both are supplied.

Changes

• Validation (__init__.py): Added a password_and_key_file check after the existing "neither provided" guard
• Translations (strings.json, translations/en.json, translations/de.json): Added the new password_and_key_file error key
• Tests (test_validate_service_data.py): Added test_password_and_key_file_raises
• Docs (README.md): Updated validation rules to reflect the mutual exclusivity of password and key_file

Original prompt

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This section details on the original issue you should resolve

<issue_title>No error when providing password AND key file</issue_title>
<issue_description>SSH Command version
v1.0.0

Home Assistant version
2026.3.2

Issue / Experienced behavior
When the action is provided with a password AND a key file, the action succeeds if the password is correct.

Expected behavior
Providing a password and a key file should always result in an error.
</issue_description>

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

• Fixes No error when providing password AND key file #14

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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 9ef8835. After a merge, _validate_service_data gained a hass parameter as its first argument, but the new test was still calling it with only data. Updated the test to pass self.mock_hass as the first argument, consistent with all other tests in the file.