tests : move save-load-state from examples to tests (#23336)

• tests : move save-load-state from examples to tests

• Move examples/save-load-state/ to tests/test-save-load-state.cpp
• Remove subdirectory reference from examples/CMakeLists.txt
• Add test to tests/CMakeLists.txt as a model test
• Remove CODEOWNERS entry for removed example directory

Assisted-by: llama.cpp:local pi

• cont : update ci