test: improve coverage for reasoning field support

- Add test for empty reasoning_content with reasoning fallback
- Add error handling tests for invalid JSON
- Coverage increased to 99.9%

Author: shitiandmw

chat_reasoning_test.go

@@ -41,6 +41,11 @@ func TestChatCompletionStreamChoiceDelta_ReasoningFieldSupport(t *testing.T) {
 			jsonData: `{"role":"assistant","content":"Hello"}`,
 			expected: "",
 		},
+		{
+			name:     "Empty reasoning_content with reasoning value",
+			jsonData: `{"role":"assistant","reasoning_content":"","reasoning":"Fallback value"}`,
+			expected: "Fallback value",
+		},
 	}
 
 	for _, tt := range tests {
@@ -58,6 +63,16 @@ func TestChatCompletionStreamChoiceDelta_ReasoningFieldSupport(t *testing.T) {
 	}
 }
 
+// TestChatCompletionStreamChoiceDelta_UnmarshalError tests error handling.
+func TestChatCompletionStreamChoiceDelta_UnmarshalError(t *testing.T) {
+	var delta openai.ChatCompletionStreamChoiceDelta
+	invalidJSON := `{"role":"assistant","content":}`
+	err := json.Unmarshal([]byte(invalidJSON), &delta)
+	if err == nil {
+		t.Error("Expected error when unmarshaling invalid JSON, got nil")
+	}
+}
+
 // TestChatCompletionMessage_ReasoningFieldSupport tests that both
 // reasoning_content and reasoning fields are properly supported in chat completion messages.
 func TestChatCompletionMessage_ReasoningFieldSupport(t *testing.T) {
@@ -92,6 +107,11 @@ func TestChatCompletionMessage_ReasoningFieldSupport(t *testing.T) {
 			jsonData: `{"role":"assistant","content":"Hello"}`,
 			expected: "",
 		},
+		{
+			name:     "Empty reasoning_content with reasoning value",
+			jsonData: `{"role":"assistant","reasoning_content":"","reasoning":"Fallback value"}`,
+			expected: "Fallback value",
+		},
 	}
 
 	for _, tt := range tests {
@@ -109,6 +129,16 @@ func TestChatCompletionMessage_ReasoningFieldSupport(t *testing.T) {
 	}
 }
 
+// TestChatCompletionMessage_UnmarshalError tests error handling.
+func TestChatCompletionMessage_UnmarshalError(t *testing.T) {
+	var msg openai.ChatCompletionMessage
+	invalidJSON := `{"role":"assistant","content":}`
+	err := json.Unmarshal([]byte(invalidJSON), &msg)
+	if err == nil {
+		t.Error("Expected error when unmarshaling invalid JSON, got nil")
+	}
+}
+
 // TestChatCompletionMessage_MultiContent_ReasoningFieldSupport tests reasoning field support
 // with MultiContent messages.
 func TestChatCompletionMessage_MultiContent_ReasoningFieldSupport(t *testing.T) {