fix(antigravity): place tool_result images in functionResponse.parts and unify mimeType

Move base64 image data from Claude tool_result into functionResponse.parts
as inlineData instead of outer sibling parts, preventing context bloat.
Unify all inlineData field naming to camelCase mimeType across Claude,
OpenAI, and Gemini translators. Add comprehensive edge case tests and
Gemini-side regression test for functionResponse.parts preservation.

internal/translator/antigravity/gemini/antigravity_gemini_request_test.go

@@ -93,3 +93,81 @@ func TestConvertGeminiRequestToAntigravity_ParallelFunctionCalls(t *testing.T) {
 		}
 	}
 }
+
+func TestFixCLIToolResponse_PreservesFunctionResponseParts(t *testing.T) {
+	// When functionResponse contains a "parts" field with inlineData (from Claude
+	// translator's image embedding), fixCLIToolResponse should preserve it as-is.
+	// parseFunctionResponseRaw returns response.Raw for valid JSON objects,
+	// so extra fields like "parts" survive the pipeline.
+	input := `{
+		"model": "claude-opus-4-6-thinking",
+		"request": {
+			"contents": [
+				{
+					"role": "model",
+					"parts": [
+						{
+							"functionCall": {"name": "screenshot", "args": {}}
+						}
+					]
+				},
+				{
+					"role": "function",
+					"parts": [
+						{
+							"functionResponse": {
+								"id": "tool-001",
+								"name": "screenshot",
+								"response": {"result": "Screenshot taken"},
+								"parts": [
+									{"inlineData": {"mimeType": "image/png", "data": "iVBOR"}}
+								]
+							}
+						}
+					]
+				}
+			]
+		}
+	}`
+
+	result, err := fixCLIToolResponse(input)
+	if err != nil {
+		t.Fatalf("fixCLIToolResponse failed: %v", err)
+	}
+
+	// Find the function response content (role=function)
+	contents := gjson.Get(result, "request.contents").Array()
+	var funcContent gjson.Result
+	for _, c := range contents {
+		if c.Get("role").String() == "function" {
+			funcContent = c
+			break
+		}
+	}
+	if !funcContent.Exists() {
+		t.Fatal("function role content should exist in output")
+	}
+
+	// The functionResponse should be preserved with its parts field
+	funcResp := funcContent.Get("parts.0.functionResponse")
+	if !funcResp.Exists() {
+		t.Fatal("functionResponse should exist in output")
+	}
+
+	// Verify the parts field with inlineData is preserved
+	inlineParts := funcResp.Get("parts").Array()
+	if len(inlineParts) != 1 {
+		t.Fatalf("Expected 1 inlineData part in functionResponse.parts, got %d", len(inlineParts))
+	}
+	if inlineParts[0].Get("inlineData.mimeType").String() != "image/png" {
+		t.Errorf("Expected mimeType 'image/png', got '%s'", inlineParts[0].Get("inlineData.mimeType").String())
+	}
+	if inlineParts[0].Get("inlineData.data").String() != "iVBOR" {
+		t.Errorf("Expected data 'iVBOR', got '%s'", inlineParts[0].Get("inlineData.data").String())
+	}
+
+	// Verify response.result is also preserved
+	if funcResp.Get("response.result").String() != "Screenshot taken" {
+		t.Errorf("Expected response.result 'Screenshot taken', got '%s'", funcResp.Get("response.result").String())
+	}
+}