chore: remove iFlow-related modules and dependencies

- Deleted `iflow` provider implementation, including thinking configuration (`apply.go`) and authentication modules.
- Removed iFlow-specific tests, executors, and helpers across SDK and internal components.
- Updated all references to exclude iFlow functionality.

internal/api/handlers/management/oauth_sessions.go

@@ -225,8 +225,6 @@ func NormalizeOAuthProvider(provider string) (string, error) {
 		return "codex", nil
 	case "gemini", "google":
 		return "gemini", nil
-	case "iflow", "i-flow":
-		return "iflow", nil
 	case "antigravity", "anti-gravity":
 		return "antigravity", nil
 	default: