[Feature]: Add session persistence for llama.cpp

#443 added session persistence for Gemini, but other providers were left unimplemented.

We should add similar implementation of Initialize() to load chat history for llama.cpp. This can happen in gollm/llamacpp.go and will likely follow a similar flow to Gemini, where we convert api messages to the provider-specific messages and add them to the history.

Adding tests is also very welcomed. Basic flows like saving and loading sessions, creating a new session, deleting a session, and listing sessions should be covered. Edge cases like invalid session ID and corrupted save file should also be covered.