Fix Gemini 3 on OpenRouter (#43416)

Release Notes:

- Gemini 3 now works on OpenRouter in the Agent Panel

crates/assistant_text_thread/src/text_thread.rs

@@ -1417,6 +1417,7 @@ impl TextThread {
                 role: Role::User,
                 content: vec!["Respond only with OK, nothing else.".into()],
                 cache: false,
+                reasoning_details: None,
             });
             req
         };
@@ -2085,6 +2086,11 @@ impl TextThread {
                                         );
                                     }
                                     LanguageModelCompletionEvent::StartMessage { .. } => {}
+                                    LanguageModelCompletionEvent::ReasoningDetails(_) => {
+                                        // ReasoningDetails are metadata (signatures, encrypted data, format info)
+                                        // used for request/response validation, not UI content.
+                                        // The displayable thinking text is already handled by the Thinking event.
+                                    }
                                     LanguageModelCompletionEvent::Stop(reason) => {
                                         stop_reason = reason;
                                     }
@@ -2308,6 +2314,7 @@ impl TextThread {
                 role: message.role,
                 content: Vec::new(),
                 cache: message.cache.as_ref().is_some_and(|cache| cache.is_anchor),
+                reasoning_details: None,
             };
 
             while let Some(content) = contents.peek() {
@@ -2679,6 +2686,7 @@ impl TextThread {
                 role: Role::User,
                 content: vec![SUMMARIZE_THREAD_PROMPT.into()],
                 cache: false,
+                reasoning_details: None,
             });
 
             // If there is no summary, it is set with `done: false` so that "Loading Summary…" can