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/utils.rs

@@ -17,19 +17,30 @@ pub fn convert_matrix_message_to_llm_message(
 
 fn convert_bot_message(matrix_message: &MatrixMessage) -> Option<Message> {
     match matrix_message.message_type {
-        MatrixMessageType::Text => convert_bot_text_message(&matrix_message.message_text),
-        MatrixMessageType::Notice => convert_bot_notice_message(&matrix_message.message_text),
+        MatrixMessageType::Text => {
+            convert_bot_text_message(&matrix_message.message_text, &matrix_message.timestamp)
+        }
+        MatrixMessageType::Notice => {
+            convert_bot_notice_message(&matrix_message.message_text, &matrix_message.timestamp)
+        }
     }
 }
 
-fn convert_bot_text_message(text: &str) -> Option<Message> {
+fn convert_bot_text_message(
+    text: &str,
+    timestamp: &chrono::DateTime<chrono::Utc>,
+) -> Option<Message> {
     Some(Message {
         author: Author::Assistant,
         message_text: text.to_owned(),
+        timestamp: timestamp.to_owned(),
     })
 }
 
-fn convert_bot_notice_message(text: &str) -> Option<Message> {
+fn convert_bot_notice_message(
+    text: &str,
+    timestamp: &chrono::DateTime<chrono::Utc>,
+) -> Option<Message> {
     // Notice messages sent by the bot are usually transcriptions of previous messages sent by the user.
     // Such transcriptions are prefixed with an emoji and blockquoted.
     // If we find a notice that doesn't match this pattern, we skip it.
@@ -42,6 +53,7 @@ fn convert_bot_notice_message(text: &str) -> Option<Message> {
         return Some(Message {
             author: Author::User,
             message_text: text.to_owned(),
+            timestamp: timestamp.to_owned(),
         });
     }
 
@@ -52,5 +64,6 @@ fn convert_user_message(matrix_message: &MatrixMessage) -> Option<Message> {
     Some(Message {
         author: Author::User,
         message_text: matrix_message.message_text.clone(),
+        timestamp: matrix_message.timestamp.to_owned(),
     })
 }