fix(openai): avoid null content when converting chat-completions to responses

When a chat-completions message has no usable content parts (empty array,
empty text part, or filtered-out image part), marshalChatInputContent
marshalled a nil slice to JSON null. The upstream Responses API rejects a
null content field with HTTP 400. Fall back to an empty string instead.

Fixes #2515

backend/internal/pkg/apicompat/chatcompletions_responses_test.go

@@ -225,6 +225,41 @@ func TestChatCompletionsToResponses_WhitespaceOnlyBase64ImageURLSkipped(t *testi
 	assert.Equal(t, "Describe this", parts[0].Text)
 }
 
+func TestChatCompletionsToResponses_EmptyContentNeverNull(t *testing.T) {
+	// Regression for #2515: the upstream Responses API rejects an input item
+	// whose content field is JSON null. Any chat-completions message that
+	// yields no usable content parts must serialize content as a string.
+	cases := []struct {
+		name    string
+		content json.RawMessage
+	}{
+		{"null content", json.RawMessage(`null`)},
+		{"empty array content", json.RawMessage(`[]`)},
+		{"only empty text part", json.RawMessage(`[{"type":"text","text":""}]`)},
+		{"only empty base64 image part", json.RawMessage(`[{"type":"image_url","image_url":{"url":"data:image/png;base64,"}}]`)},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			req := &ChatCompletionsRequest{
+				Model: "gpt-5.5",
+				Messages: []ChatMessage{
+					{Role: "user", Content: tc.content},
+				},
+			}
+			resp, err := ChatCompletionsToResponses(req)
+			require.NoError(t, err)
+			assert.NotContains(t, string(resp.Input), `"content":null`,
+				"converted input must not contain a null content field")
+
+			var items []ResponsesInputItem
+			require.NoError(t, json.Unmarshal(resp.Input, &items))
+			require.Len(t, items, 1)
+			assert.Equal(t, `""`, string(items[0].Content),
+				"content must be an empty string, not null")
+		})
+	}
+}
+
 func TestChatCompletionsToResponses_SystemArrayContent(t *testing.T) {
 	req := &ChatCompletionsRequest{
 		Model: "gpt-4o",

backend/internal/pkg/apicompat/chatcompletions_to_responses.go

@@ -325,7 +325,14 @@ func marshalChatInputContent(content chatMessageContent) (json.RawMessage, error
 	if content.Text != nil {
 		return json.Marshal(*content.Text)
 	}
-	return json.Marshal(convertChatContentPartsToResponses(content.Parts))
+	parts := convertChatContentPartsToResponses(content.Parts)
+	if len(parts) == 0 {
+		// A nil slice marshals to JSON null, which the upstream Responses API
+		// rejects ("expected an array of objects or string, but got null").
+		// Fall back to an empty string when no usable parts remain.
+		return json.Marshal("")
+	}
+	return json.Marshal(parts)
 }
 
 func convertChatContentPartsToResponses(parts []ChatContentPart) []ResponsesContentPart {