Add think field to ChatRequest

The Ollama API supports the think parameter for both generate and
chat endpoints. ChatRequest was missing it while GenerateRequest
already had it. Added the field, builder method, and doc comment
to bring chat to parity with generate.

src/types/chat.rs

@@ -2,7 +2,7 @@ use serde::{Deserialize, Serialize};
 use serde_json::Value;
 
 use crate::error::OllamaResult;
-use crate::types::common::Options;
+use crate::types::common::{Options, Think};
 
 #[derive(Clone, Debug, Serialize, Deserialize)]
 #[serde(rename_all = "lowercase")]
@@ -66,6 +66,11 @@ pub struct ChatRequest {
 
     #[serde(skip_serializing_if = "Option::is_none")]
     pub format: Option<Value>,
+
+    /// When set, returns separate thinking output in addition to content. Can be a boolean
+    /// (true/false) or a string ("high", "medium", "low") for supported models.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub think: Option<Think>,
 }
 
 impl ChatRequest {
@@ -96,6 +101,7 @@ impl ChatRequestBuilder {
                 options: None,
                 tools: vec![],
                 format: None,
+                think: None,
             },
         }
     }
@@ -125,6 +131,11 @@ impl ChatRequestBuilder {
         self
     }
 
+    pub fn think(mut self, think: Think) -> Self {
+        self.chat_request.think = Some(think);
+        self
+    }
+
     pub fn build(self) -> ChatRequest {
         self.chat_request
     }