feat(auth): centralize token store management and enhance persistence

- Introduced `RegisterTokenStore` and `GetTokenStore` to centralize token store access.
- Replaced direct file operations with a unified token persistence API.
- Updated all components to use the shared token store for consistent behavior.
- Improved logging for token save operations to include file paths.

sdk/auth/store_registry.go

@@ -0,0 +1,31 @@
+package auth
+
+import "sync"
+
+var (
+	storeMu              sync.RWMutex
+	registeredTokenStore TokenStore
+)
+
+// RegisterTokenStore sets the global token store used by the authentication helpers.
+func RegisterTokenStore(store TokenStore) {
+	storeMu.Lock()
+	registeredTokenStore = store
+	storeMu.Unlock()
+}
+
+// GetTokenStore returns the globally registered token store.
+func GetTokenStore() TokenStore {
+	storeMu.RLock()
+	s := registeredTokenStore
+	storeMu.RUnlock()
+	if s != nil {
+		return s
+	}
+	storeMu.Lock()
+	defer storeMu.Unlock()
+	if registeredTokenStore == nil {
+		registeredTokenStore = NewFileTokenStore()
+	}
+	return registeredTokenStore
+}