refactor(thinking): simplify ModeNone and budget validation logic

internal/thinking/validate.go

@@ -159,27 +159,22 @@ func ValidateConfig(config ThinkingConfig, modelInfo *registry.ModelInfo, provid
 		normalized = convertAutoToMidRange(normalized, support, provider, model)
 	}
 
-	// ModeNone handling varies by provider/model capability:
-	// - Claude can always disable thinking via thinking.type="disabled".
-	// - Some models/providers cannot be fully disabled; for those we force minimal thinking
-	//   while hiding thoughts (ModeNone + Budget>0 and/or Level set).
-	if normalized.Mode == ModeNone {
-		if provider == "claude" {
-			normalized.Budget = 0
-			normalized.Level = ""
-		} else if !support.ZeroAllowed {
-			if support.Min > 0 {
-				normalized.Budget = support.Min
-			}
-			if normalized.Level == "" && normalized.Budget > 0 && len(support.Levels) > 0 {
-				normalized.Level = ThinkingLevel(support.Levels[0])
-			}
+	if normalized.Mode == ModeNone && provider == "claude" {
+		// Claude supports explicit disable via thinking.type="disabled".
+		// Keep Budget=0 so applier can omit budget_tokens.
+		normalized.Budget = 0
+		normalized.Level = ""
+	} else {
+		switch normalized.Mode {
+		case ModeBudget, ModeAuto, ModeNone:
+			normalized.Budget = ClampBudget(normalized.Budget, modelInfo, provider)
 		}
-	}
 
-	switch normalized.Mode {
-	case ModeBudget, ModeAuto:
-		normalized.Budget = ClampBudget(normalized.Budget, modelInfo, provider)
+		// ModeNone with clamped Budget > 0: set Level to lowest for Level-only/Hybrid models
+		// This ensures Apply layer doesn't need to access support.Levels
+		if normalized.Mode == ModeNone && normalized.Budget > 0 && len(support.Levels) > 0 {
+			normalized.Level = ThinkingLevel(support.Levels[0])
+		}
 	}
 
 	return &normalized, nil