agent: Do not create user messages for tool results in thread (#29354)

We used to insert empty user messages into the `Thread::messages` `Vec`
when tools finished running and then we would attach the results when
creating the request. This approach was very easy to mess up during
state handling, leading to empty user messages displayed in the
conversation and API failures.

Instead, we will no longer insert actual user messages for tool results
to the `Thread`, and will only do this on the fly when creating the
model request. This simplifies a lot of code and show fix the mentioned
errors.

Release Notes:

- agent: Improve reliability of LLM requests when including tool results

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Oleksiy Syvokon <oleksiy.syvokon@gmail.com>

crates/agent/src/thread_store.rs

@@ -639,12 +639,17 @@ pub struct SerializedThread {
 }
 
 impl SerializedThread {
-    pub const VERSION: &'static str = "0.1.0";
+    pub const VERSION: &'static str = "0.2.0";
 
     pub fn from_json(json: &[u8]) -> Result<Self> {
         let saved_thread_json = serde_json::from_slice::<serde_json::Value>(json)?;
         match saved_thread_json.get("version") {
             Some(serde_json::Value::String(version)) => match version.as_str() {
+                SerializedThreadV0_1_0::VERSION => {
+                    let saved_thread =
+                        serde_json::from_value::<SerializedThreadV0_1_0>(saved_thread_json)?;
+                    Ok(saved_thread.upgrade())
+                }
                 SerializedThread::VERSION => Ok(serde_json::from_value::<SerializedThread>(
                     saved_thread_json,
                 )?),
@@ -666,6 +671,38 @@ impl SerializedThread {
     }
 }
 
+#[derive(Serialize, Deserialize, Debug)]
+pub struct SerializedThreadV0_1_0(
+    // The structure did not change, so we are reusing the latest SerializedThread.
+    // When making the next version, make sure this points to SerializedThreadV0_2_0
+    SerializedThread,
+);
+
+impl SerializedThreadV0_1_0 {
+    pub const VERSION: &'static str = "0.1.0";
+
+    pub fn upgrade(self) -> SerializedThread {
+        debug_assert_eq!(SerializedThread::VERSION, "0.2.0");
+
+        let mut messages: Vec<SerializedMessage> = Vec::with_capacity(self.0.messages.len());
+
+        for message in self.0.messages {
+            if message.role == Role::User && !message.tool_results.is_empty() {
+                if let Some(last_message) = messages.last_mut() {
+                    debug_assert!(last_message.role == Role::Assistant);
+
+                    last_message.tool_results = message.tool_results;
+                    continue;
+                }
+            }
+
+            messages.push(message);
+        }
+
+        SerializedThread { messages, ..self.0 }
+    }
+}
+
 #[derive(Debug, Serialize, Deserialize)]
 pub struct SerializedMessage {
     pub id: MessageId,