Avoid default service crash without bundle identifier

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Summary

• avoid force-unwrapping Bundle.main.bundleIdentifier for the default service value
• fall back to SimpleKeychain when no bundle identifier is available
• update the default initialization test for the fallback

Validation

• Not run locally: Swift toolchain is not installed on this Windows machine.
• Local checkout note: generated docs contain Windows-invalid paths, so the source was inspected from an archive excluding docs.