fix(translator): ignore null and empty finish reasons in openai to gemini response

- Check that `finish_reason` is a non-empty string before mapping to Gemini `finishReason`.
- Prevent chunks or messages with `null` or empty `finish_reason` from emitting unexpected completion statuses.

Closes: #5651

internal/translator/openai/gemini/openai_gemini_response.go

@@ -201,7 +201,7 @@ func ConvertOpenAIResponseToGemini(_ context.Context, _ string, originalRequestR
 			}
 
 			// Handle finish reason
-			if finishReason := choice.Get("finish_reason"); finishReason.Exists() {
+			if finishReason := choice.Get("finish_reason"); finishReason.Type == gjson.String && finishReason.String() != "" {
 				geminiFinishReason := mapOpenAIFinishReasonToGemini(finishReason.String())
 				template, _ = sjson.SetBytes(template, "candidates.0.finishReason", geminiFinishReason)
 
@@ -606,7 +606,7 @@ func ConvertOpenAIResponseToGeminiNonStream(_ context.Context, _ string, origina
 			}
 
 			// Handle finish reason
-			if finishReason := choice.Get("finish_reason"); finishReason.Exists() {
+			if finishReason := choice.Get("finish_reason"); finishReason.Type == gjson.String && finishReason.String() != "" {
 				geminiFinishReason := mapOpenAIFinishReasonToGemini(finishReason.String())
 				out, _ = sjson.SetBytes(out, "candidates.0.finishReason", geminiFinishReason)
 			}

internal/translator/openai/gemini/openai_gemini_response_test.go

@@ -85,3 +85,93 @@ func TestConvertOpenAIResponseToGeminiNonStream_MultiChoicePartsOverlay(t *testi
 		t.Fatalf("expected functionCall.id to be 'call_2', got %q", parts3[0].Get("functionCall.id").String())
 	}
 }
+
+func TestConvertOpenAIResponseToGeminiStream_NullFinishReasonIgnored(t *testing.T) {
+	var param any
+
+	// Chunk 1: Contentless delta with explicit finish_reason: null (e.g. role declaration or pre-reasoning delta)
+	chunk1 := []byte(`{"choices":[{"index":0,"delta":{"role":"assistant","content":""},"finish_reason":null}]}`)
+	out1 := ConvertOpenAIResponseToGemini(context.Background(), "gpt-test", nil, nil, chunk1, &param)
+	for i, chunk := range out1 {
+		if fr := gjson.GetBytes(chunk, "candidates.0.finishReason"); fr.Exists() && fr.String() != "" {
+			t.Fatalf("chunk1[%d] unexpectedly set finishReason = %q on non-final chunk; payload=%s", i, fr.String(), chunk)
+		}
+	}
+
+	// Chunk 2: Reasoning delta with finish_reason: null
+	chunk2 := []byte(`{"choices":[{"index":0,"delta":{"reasoning_content":"thinking..."},"finish_reason":null}]}`)
+	out2 := ConvertOpenAIResponseToGemini(context.Background(), "gpt-test", nil, nil, chunk2, &param)
+	if len(out2) == 0 {
+		t.Fatalf("expected output for reasoning chunk, got 0 chunks")
+	}
+	if gotThought := gjson.GetBytes(out2[0], "candidates.0.content.parts.0.thought").Bool(); !gotThought {
+		t.Fatalf("expected thought: true on reasoning chunk, got false")
+	}
+	if gotText := gjson.GetBytes(out2[0], "candidates.0.content.parts.0.text").String(); gotText != "thinking..." {
+		t.Fatalf("expected reasoning text %q, got %q", "thinking...", gotText)
+	}
+	for i, chunk := range out2 {
+		if fr := gjson.GetBytes(chunk, "candidates.0.finishReason"); fr.Exists() && fr.String() != "" {
+			t.Fatalf("chunk2[%d] unexpectedly set finishReason = %q on reasoning chunk; payload=%s", i, fr.String(), chunk)
+		}
+	}
+
+	// Chunk 3: Contentless delta with finish_reason: "" (empty string should not be treated as stop)
+	chunk3 := []byte(`{"choices":[{"index":0,"delta":{},"finish_reason":""}]}`)
+	out3 := ConvertOpenAIResponseToGemini(context.Background(), "gpt-test", nil, nil, chunk3, &param)
+	for i, chunk := range out3 {
+		if fr := gjson.GetBytes(chunk, "candidates.0.finishReason"); fr.Exists() && fr.String() != "" {
+			t.Fatalf("chunk3[%d] unexpectedly set finishReason = %q on contentless chunk with empty finish_reason; payload=%s", i, fr.String(), chunk)
+		}
+	}
+
+	// Chunk 4: Content delta with finish_reason: null
+	chunk4 := []byte(`{"choices":[{"index":0,"delta":{"content":"hello world"},"finish_reason":null}]}`)
+	out4 := ConvertOpenAIResponseToGemini(context.Background(), "gpt-test", nil, nil, chunk4, &param)
+	if len(out4) == 0 {
+		t.Fatalf("expected output for content chunk, got 0 chunks")
+	}
+	if gotText := gjson.GetBytes(out4[0], "candidates.0.content.parts.0.text").String(); gotText != "hello world" {
+		t.Fatalf("expected content text %q, got %q", "hello world", gotText)
+	}
+	for i, chunk := range out4 {
+		if fr := gjson.GetBytes(chunk, "candidates.0.finishReason"); fr.Exists() && fr.String() != "" {
+			t.Fatalf("chunk4[%d] unexpectedly set finishReason = %q on non-final content chunk; payload=%s", i, fr.String(), chunk)
+		}
+	}
+
+	// Chunk 5: Final chunk with finish_reason: "stop"
+	chunk5 := []byte(`{"choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}`)
+	out5 := ConvertOpenAIResponseToGemini(context.Background(), "gpt-test", nil, nil, chunk5, &param)
+	if len(out5) == 0 {
+		t.Fatalf("expected output for final chunk, got 0 chunks")
+	}
+	if got := gjson.GetBytes(out5[len(out5)-1], "candidates.0.finishReason").String(); got != "STOP" {
+		t.Fatalf("expected finishReason STOP on final chunk, got %q", got)
+	}
+}
+
+func TestConvertOpenAIResponseToGeminiNonStream_NullFinishReasonIgnored(t *testing.T) {
+	testCases := []struct {
+		name    string
+		payload []byte
+	}{
+		{
+			name:    "null finish_reason",
+			payload: []byte(`{"choices":[{"index":0,"message":{"role":"assistant","content":"hello"},"finish_reason":null}]}`),
+		},
+		{
+			name:    "empty finish_reason",
+			payload: []byte(`{"choices":[{"index":0,"message":{"role":"assistant","content":"hello"},"finish_reason":""}]}`),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			out := ConvertOpenAIResponseToGeminiNonStream(context.Background(), "gpt-test", nil, nil, tc.payload, nil)
+			if fr := gjson.GetBytes(out, "candidates.0.finishReason"); fr.Exists() && fr.String() != "" {
+				t.Fatalf("unexpected finishReason = %q on non-stream response; output=%s", fr.String(), out)
+			}
+		})
+	}
+}