fix: support response parts with extra fields like thoughtSignature

Implement custom Deserialize for Part to tolerate additional response fields (such as thoughtSignature or thought) returned by thinking models.

src/types/common.rs

@@ -83,7 +83,7 @@ impl FromStr for Role {
     }
 }
 
-#[derive(Clone, Debug, Serialize, Deserialize)]
+#[derive(Clone, Debug, Serialize)]
 #[serde(rename_all = "camelCase")]
 pub enum Part {
     Text(String),
@@ -100,3 +100,66 @@ pub enum Part {
         args: HashMap<String, String>,
     },
 }
+
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct PartHelper {
+    text: Option<String>,
+    inline_data: Option<InlineDataHelper>,
+    file_data: Option<FileDataHelper>,
+    function_call: Option<FunctionCallHelper>,
+}
+
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct InlineDataHelper {
+    mime_type: String,
+    data: String,
+}
+
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct FileDataHelper {
+    mime_type: String,
+    file_uri: String,
+}
+
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct FunctionCallHelper {
+    name: String,
+    args: HashMap<String, String>,
+}
+
+impl<'de> Deserialize<'de> for Part {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let helper = PartHelper::deserialize(deserializer)?;
+        if let Some(text) = helper.text {
+            return Ok(Part::Text(text));
+        }
+        if let Some(inline_data) = helper.inline_data {
+            return Ok(Part::InlineData {
+                mime_type: inline_data.mime_type,
+                data: inline_data.data,
+            });
+        }
+        if let Some(file_data) = helper.file_data {
+            return Ok(Part::FileData {
+                mime_type: file_data.mime_type,
+                file_uri: file_data.file_uri,
+            });
+        }
+        if let Some(function_call) = helper.function_call {
+            return Ok(Part::FunctionCall {
+                name: function_call.name,
+                args: function_call.args,
+            });
+        }
+        Err(serde::de::Error::custom(
+            "Part does not contain any recognizable variant",
+        ))
+    }
+}

src/types/generate_content.rs

@@ -531,4 +531,34 @@ mod tests {
         }"#;
         serde_json::from_str::<GenerateContentResponse>(input).unwrap();
     }
+
+    #[test]
+    fn parses_response_with_thought_signature() {
+        let input = r#"{
+          "candidates": [
+            {
+              "content": {
+                "role": "model",
+                "parts": [
+                  {
+                    "text": "Hello world",
+                    "thoughtSignature": "AY89a1+twFsQ+oYDReN5PU76yp0ciDinNh3MgyCbe/BLvyh93lje7qHOCJuNxaPULnqmgvKvtuQnjyf2wwD20Dl2rWnxbdZZAeqhRJEdvKOc4LQ="
+                  }
+                ]
+              },
+              "finishReason": "STOP"
+            }
+          ],
+          "usageMetadata": {
+            "promptTokenCount": 877,
+            "candidatesTokenCount": 547,
+            "totalTokenCount": 1424,
+            "trafficType": "ON_DEMAND"
+          },
+          "modelVersion": "gemini-3.5-flash-lite",
+          "createTime": "2026-09-06T10:36:16.330983Z",
+          "responseId": "IEKdaueZFKyrz_IPp8Sc4QQ"
+        }"#;
+        serde_json::from_str::<GenerateContentResponse>(input).unwrap();
+    }
 }