Allow for prompt caching to work by using baibot_conversation_start_time_utc instead of baibot_now_utc

This patch introduces a new `baibot_conversation_start_time_utc`
variable which indicates the time the conversation got started.

Using `baibot_now_utc` is still possible, but given that the current
time is a moving target, its use is in conflict with prompt caching.

Because the new `baibot_conversation_start_time_utc` prompt variable
is a more reasonable default, we're now using it in all sample configs.

src/conversation/llm/entity.rs

@@ -1,3 +1,5 @@
+use chrono::{DateTime, Utc};
+
 #[derive(Debug, Clone, PartialEq)]
 pub enum Author {
     Prompt,
@@ -9,6 +11,7 @@ pub enum Author {
 pub struct Message {
     pub author: Author,
     pub message_text: String,
+    pub timestamp: DateTime<Utc>,
 }
 
 #[derive(Debug)]
@@ -52,39 +55,59 @@ impl Conversation {
             messages: new_messages,
         }
     }
+
+    pub fn start_time(&self) -> Option<DateTime<Utc>> {
+        self.messages.first().map(|message| message.timestamp)
+    }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
+    use chrono::{TimeZone, Utc};
 
     #[test]
     fn combine_consecutive_messages() {
+        let timestamp_1 = Utc.with_ymd_and_hms(2024, 9, 20, 18, 34, 15).unwrap();
+
+        let timestamp_2 = Utc.with_ymd_and_hms(2024, 9, 21, 18, 34, 15).unwrap();
+
+        let timestamp_3 = Utc.with_ymd_and_hms(2024, 9, 22, 18, 34, 15).unwrap();
+
         let conversation = Conversation {
             messages: vec![
+                // User's turn
                 Message {
                     author: Author::User,
                     message_text: "Hello".to_string(),
+                    timestamp: timestamp_1,
                 },
                 Message {
                     author: Author::User,
                     message_text: "How are you?".to_string(),
+                    timestamp: timestamp_2,
                 },
                 Message {
                     author: Author::User,
                     message_text: "I'm OK, btw.".to_string(),
+                    timestamp: timestamp_3,
                 },
+                // Assistant's turn
                 Message {
                     author: Author::Assistant,
                     message_text: "Hi there!".to_string(),
+                    timestamp: timestamp_2,
                 },
                 Message {
                     author: Author::Assistant,
                     message_text: "I'm doing well, thank you.".to_string(),
+                    timestamp: timestamp_3,
                 },
+                // User's turn
                 Message {
                     author: Author::User,
                     message_text: "That's great!".to_string(),
+                    timestamp: timestamp_3,
                 },
             ],
         };
@@ -97,12 +120,17 @@ mod tests {
             conversation.messages[0].message_text,
             "Hello\nHow are you?\nI'm OK, btw."
         );
+        assert_eq!(conversation.messages[0].timestamp, timestamp_1);
+
         assert_eq!(conversation.messages[1].author, Author::Assistant);
         assert_eq!(
             conversation.messages[1].message_text,
             "Hi there!\nI'm doing well, thank you."
         );
+        assert_eq!(conversation.messages[1].timestamp, timestamp_2);
+
         assert_eq!(conversation.messages[2].author, Author::User);
         assert_eq!(conversation.messages[2].message_text, "That's great!");
+        assert_eq!(conversation.messages[2].timestamp, timestamp_3);
     }
 }