language_models: Add thinking support to DeepSeek provider (#32338)

For DeepSeek provider thinking is returned as reasoning_content and we
don't have to send the reasoning_content back in the request.

Release Notes:

- Add thinking support to DeepSeek provider

crates/language_models/src/provider/deepseek.rs

@@ -372,8 +372,7 @@ pub fn into_deepseek(
     for message in request.messages {
         for content in message.content {
             match content {
-                MessageContent::Text(text) | MessageContent::Thinking { text, .. } => messages
+                MessageContent::Text(text) => messages.push(match message.role {
-                    .push(match message.role {
                     Role::User => deepseek::RequestMessage::User { content: text },
                     Role::Assistant => deepseek::RequestMessage::Assistant {
                         content: Some(text),
@@ -381,6 +380,7 @@ pub fn into_deepseek(
                     },
                     Role::System => deepseek::RequestMessage::System { content: text },
                 }),
+                MessageContent::Thinking { .. } => {}
                 MessageContent::RedactedThinking(_) => {}
                 MessageContent::Image(_) => {}
                 MessageContent::ToolUse(tool_use) => {
@@ -485,6 +485,13 @@ impl DeepSeekEventMapper {
             events.push(Ok(LanguageModelCompletionEvent::Text(content)));
         }
 
+        if let Some(reasoning_content) = choice.delta.reasoning_content.clone() {
+            events.push(Ok(LanguageModelCompletionEvent::Thinking {
+                text: reasoning_content,
+                signature: None,
+            }));
+        }
+
         if let Some(tool_calls) = choice.delta.tool_calls.as_ref() {
             for tool_call in tool_calls {
                 let entry = self.tool_calls_by_index.entry(tool_call.index).or_default();