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/thinking/strip.go

@@ -44,13 +44,6 @@ func StripThinkingConfig(body []byte, provider string) []byte {
 		}
 	case "codex":
 		paths = []string{"reasoning.effort"}
-	case "iflow":
-		paths = []string{
-			"chat_template_kwargs.enable_thinking",
-			"chat_template_kwargs.clear_thinking",
-			"reasoning_split",
-			"reasoning_effort",
-		}
 	default:
 		return body
 	}