test(parser): normalize failed-none invalid users

[stacknil]

Summary

• normalize OpenSSH Failed none for invalid user ... probing into the existing ssh_invalid_user event family
• keep Failed none for non-invalid users unsupported so the parser does not invent a new failure family
• update parser tests, fixture matrices, and parser contract documentation

Validation

• cmake --build build --config Debug --target test_parser
• ctest --test-dir build -C Debug -R parser --output-on-failure
• cmake --build build --config Debug
• ctest --test-dir build -C Debug --output-on-failure
• git diff --check

Safety

• Fixture additions use example-host and 203.0.113.x documentation addresses only.
• No new detector surface or event enum was added.