feat(thinking): rename isBudgetBasedProvider to isBudgetCapableProvider and update logic for provider checks

internal/thinking/apply.go

@@ -293,7 +293,7 @@ func normalizeUserDefinedConfig(config ThinkingConfig, fromFormat, toFormat stri
 	if config.Mode != ModeLevel {
 		return config
 	}
-	if !isBudgetBasedProvider(toFormat) || !isLevelBasedProvider(fromFormat) {
+	if !isBudgetCapableProvider(toFormat) {
 		return config
 	}
 	budget, ok := ConvertLevelToBudget(string(config.Level))

internal/thinking/validate.go

@@ -335,7 +335,9 @@ func normalizeLevels(levels []string) []string {
 	return out
 }
 
-func isBudgetBasedProvider(provider string) bool {
+// isBudgetCapableProvider returns true if the provider supports budget-based thinking.
+// These providers may also support level-based thinking (hybrid models).
+func isBudgetCapableProvider(provider string) bool {
 	switch provider {
 	case "gemini", "gemini-cli", "antigravity", "claude":
 		return true
@@ -344,15 +346,6 @@ func isBudgetBasedProvider(provider string) bool {
 	}
 }
 
-func isLevelBasedProvider(provider string) bool {
-	switch provider {
-	case "openai", "openai-response", "codex":
-		return true
-	default:
-		return false
-	}
-}
-
 func isGeminiFamily(provider string) bool {
 	switch provider {
 	case "gemini", "gemini-cli", "antigravity":