refactor(thinking): pass source and target formats to ApplyThinking for cross-format validation

Update ApplyThinking signature to accept fromFormat and toFormat parameters
instead of a single provider string. This enables:

- Proper level-to-budget conversion when source is level-based (openai/codex)
  and target is budget-based (gemini/claude)
- Strict budget range validation when source and target formats match
- Level clamping to nearest supported level for cross-format requests
- Format alias resolution in SDK translator registry for codex/openai-response

Also adds ErrBudgetOutOfRange error code and improves iflow config extraction
to fall back to openai format when iflow-specific config is not present.

internal/thinking/strip.go

@@ -27,28 +27,32 @@ func StripThinkingConfig(body []byte, provider string) []byte {
 		return body
 	}
 
+	var paths []string
 	switch provider {
 	case "claude":
-		result, _ := sjson.DeleteBytes(body, "thinking")
-		return result
+		paths = []string{"thinking"}
 	case "gemini":
-		result, _ := sjson.DeleteBytes(body, "generationConfig.thinkingConfig")
-		return result
+		paths = []string{"generationConfig.thinkingConfig"}
 	case "gemini-cli", "antigravity":
-		result, _ := sjson.DeleteBytes(body, "request.generationConfig.thinkingConfig")
-		return result
+		paths = []string{"request.generationConfig.thinkingConfig"}
 	case "openai":
-		result, _ := sjson.DeleteBytes(body, "reasoning_effort")
-		return result
+		paths = []string{"reasoning_effort"}
 	case "codex":
-		result, _ := sjson.DeleteBytes(body, "reasoning.effort")
-		return result
+		paths = []string{"reasoning.effort"}
 	case "iflow":
-		result, _ := sjson.DeleteBytes(body, "chat_template_kwargs.enable_thinking")
-		result, _ = sjson.DeleteBytes(result, "chat_template_kwargs.clear_thinking")
-		result, _ = sjson.DeleteBytes(result, "reasoning_split")
-		return result
+		paths = []string{
+			"chat_template_kwargs.enable_thinking",
+			"chat_template_kwargs.clear_thinking",
+			"reasoning_split",
+			"reasoning_effort",
+		}
 	default:
 		return body
 	}
+
+	result := body
+	for _, path := range paths {
+		result, _ = sjson.DeleteBytes(result, path)
+	}
+	return result
 }