Fix Gemini 3 on OpenRouter (#43416)

Release Notes:

- Gemini 3 now works on OpenRouter in the Agent Panel

crates/copilot/src/copilot_chat.rs

@@ -353,6 +353,8 @@ pub enum ToolCallContent {
 pub struct FunctionContent {
     pub name: String,
     pub arguments: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub thought_signature: Option<String>,
 }
 
 #[derive(Deserialize, Debug)]
@@ -396,6 +398,7 @@ pub struct ToolCallChunk {
 pub struct FunctionChunk {
     pub name: Option<String>,
     pub arguments: Option<String>,
+    pub thought_signature: Option<String>,
 }
 
 #[derive(Deserialize)]

crates/copilot/src/copilot_responses.rs

@@ -127,6 +127,8 @@ pub enum ResponseInputItem {
         arguments: String,
         #[serde(skip_serializing_if = "Option::is_none")]
         status: Option<ItemStatus>,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        thought_signature: Option<String>,
     },
     FunctionCallOutput {
         call_id: String,
@@ -251,6 +253,8 @@ pub enum ResponseOutputItem {
         arguments: String,
         #[serde(skip_serializing_if = "Option::is_none")]
         status: Option<ItemStatus>,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        thought_signature: Option<String>,
     },
     Reasoning {
         id: String,