refactor(auth): replace TokenRecord with coreauth.Auth and migrate TokenStore to coreauth.Store

- Replaced `TokenRecord` with `coreauth.Auth` for centralized and consistent authentication data structures.
- Migrated `TokenStore` interface to `coreauth.Store` for alignment with core CLIProxy authentication.
- Updated related login methods, token persistence logic, and file storage handling to use the new `coreauth.Auth` model.

examples/custom-provider/main.go

@@ -160,11 +160,7 @@ func main() {
 	if dirSetter, ok := tokenStore.(interface{ SetBaseDir(string) }); ok {
 		dirSetter.SetBaseDir(cfg.AuthDir)
 	}
-	store, ok := tokenStore.(coreauth.Store)
-	if !ok {
-		panic("token store does not implement coreauth.Store")
-	}
-	core := coreauth.NewManager(store, nil, nil)
+	core := coreauth.NewManager(tokenStore, nil, nil)
 	core.RegisterExecutor(MyExecutor{})
 
 	hooks := cliproxy.Hooks{