Fix Gemini 3 on OpenRouter (#43416)

Release Notes:

- Gemini 3 now works on OpenRouter in the Agent Panel

crates/language_model/src/language_model.rs

@@ -98,6 +98,7 @@ pub enum LanguageModelCompletionEvent {
     StartMessage {
         message_id: String,
     },
+    ReasoningDetails(serde_json::Value),
     UsageUpdate(TokenUsage),
 }
 
@@ -680,6 +681,7 @@ pub trait LanguageModel: Send + Sync {
                                 Ok(LanguageModelCompletionEvent::Text(text)) => Some(Ok(text)),
                                 Ok(LanguageModelCompletionEvent::Thinking { .. }) => None,
                                 Ok(LanguageModelCompletionEvent::RedactedThinking { .. }) => None,
+                                Ok(LanguageModelCompletionEvent::ReasoningDetails(_)) => None,
                                 Ok(LanguageModelCompletionEvent::Stop(_)) => None,
                                 Ok(LanguageModelCompletionEvent::ToolUse(_)) => None,
                                 Ok(LanguageModelCompletionEvent::ToolUseJsonParseError {
@@ -1034,8 +1036,8 @@ mod tests {
         let original = LanguageModelToolUse {
             id: LanguageModelToolUseId::from("no_sig_id"),
             name: "no_sig_tool".into(),
-            raw_input: json!({"key": "value"}).to_string(),
-            input: json!({"key": "value"}),
+            raw_input: json!({"arg": "value"}).to_string(),
+            input: json!({"arg": "value"}),
             is_input_complete: true,
             thought_signature: None,
         };

crates/language_model/src/request.rs

@@ -357,6 +357,8 @@ pub struct LanguageModelRequestMessage {
     pub role: Role,
     pub content: Vec<MessageContent>,
     pub cache: bool,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub reasoning_details: Option<serde_json::Value>,
 }
 
 impl LanguageModelRequestMessage {