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/tokenization.rs

@@ -86,6 +86,7 @@ pub mod test {
         let message = super::Message {
             author: super::Author::User,
             message_text: "Hello there!".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
 
         let tokens = super::calculate_token_size_for_message(&bpe, model, &message);
@@ -104,6 +105,7 @@ pub mod test {
         let prompt = super::Message {
             author: super::Author::Prompt,
             message_text: "You are a bot!".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let prompt_length = 10;
 
@@ -117,6 +119,7 @@ pub mod test {
         let first = super::Message {
             author: super::Author::User,
             message_text: "Hello there!".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let first_length = 8;
 
@@ -130,6 +133,7 @@ pub mod test {
         let second = super::Message {
             author: super::Author::Assistant,
             message_text: "Hello!".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let second_length = 7;
 
@@ -144,6 +148,7 @@ pub mod test {
             author: super::Author::User,
             message_text: "This is the 3rd message in this conversation. It shall be preserved."
                 .to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let third_length = 21;
 
@@ -157,6 +162,7 @@ pub mod test {
         let forth = super::Message {
             author: super::Author::Assistant,
             message_text: "This is yet another message that shall be preserved.".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let forth_length = 15;
 
@@ -201,6 +207,7 @@ pub mod test {
         let prompt = super::Message {
             author: super::Author::User,
             message_text: "あなたはボットです。".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let prompt_length = 14;
 
@@ -214,6 +221,7 @@ pub mod test {
         let first = super::Message {
             author: super::Author::User,
             message_text: "こんにちは!".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let first_length = 7;
 
@@ -227,6 +235,7 @@ pub mod test {
         let second = super::Message {
             author: super::Author::Assistant,
             message_text: "こんにちは。今日は元気ですか。".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let second_length = 15;
 
@@ -240,6 +249,7 @@ pub mod test {
         let third = super::Message {
             author: super::Author::User,
             message_text: "これは第3のメッセージなので、保存されます。".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let third_length = 22;
 
@@ -253,6 +263,7 @@ pub mod test {
         let forth = super::Message {
             author: super::Author::Assistant,
             message_text: "これはもう一つの保存されますメッセージです。".to_owned(),
+            timestamp: chrono::Utc::now(),
         };
         let forth_length = 21;