Add support for prompt variables (bot name, date/time, model id)

Fixes https://github.com/etkecc/baibot/issues/10

This also includes them in the default prompts (for newly-created agents),
so that people can get a better experience out of the box.

src/agent/provider/entity/text_generation/mod.rs

@@ -0,0 +1,15 @@
+mod prompt_variables;
+
+pub use prompt_variables::TextGenerationPromptVariables;
+
+#[derive(Default)]
+pub struct TextGenerationParams {
+    pub context_management_enabled: bool,
+    pub prompt_override: Option<String>,
+    pub temperature_override: Option<f32>,
+    pub prompt_variables: TextGenerationPromptVariables,
+}
+
+pub struct TextGenerationResult {
+    pub text: String,
+}

src/agent/provider/entity/text_generation/prompt_variables.rs

@@ -0,0 +1,75 @@
+use chrono::{DateTime, Utc};
+use std::collections::HashMap;
+
+pub struct TextGenerationPromptVariables {
+    map: HashMap<String, String>,
+}
+
+impl Default for TextGenerationPromptVariables {
+    fn default() -> Self {
+        Self::new("unnamed", "unknown-model", Utc::now())
+    }
+}
+
+impl TextGenerationPromptVariables {
+    pub fn new(bot_name: &str, model_id: &str, utc_time: DateTime<Utc>) -> Self {
+        let mut map = HashMap::new();
+
+        map.insert("baibot_name".to_string(), bot_name.to_string());
+        map.insert("baibot_model_id".to_string(), model_id.to_string());
+        map.insert("baibot_now_utc".to_string(), format_utc_time(utc_time));
+
+        Self { map }
+    }
+
+    pub fn format(&self, text: &str) -> String {
+        let mut formatted_text = text.to_string();
+
+        for (key, value) in &self.map {
+            let placeholder = format!("{{{{ {} }}}}", key);
+            formatted_text = formatted_text.replace(&placeholder, value);
+        }
+
+        formatted_text
+    }
+}
+
+fn format_utc_time(time: DateTime<Utc>) -> String {
+    time.format("%Y-%m-%d (%A), %H:%M:%S UTC").to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use chrono::{TimeZone, Timelike};
+
+    #[test]
+    fn test_new() {
+        // Intentionally injecting some sub-seconds to ensure formatting would ignore them.
+        let now_utc = Utc
+            .with_ymd_and_hms(2024, 9, 20, 18, 34, 15)
+            .unwrap()
+            .with_nanosecond(250000000)
+            .unwrap();
+
+        let variables = TextGenerationPromptVariables::new("baibot", "gpt-4o", now_utc);
+
+        assert_eq!(
+            variables.map.get("baibot_name"),
+            Some(&"baibot".to_string())
+        );
+        assert_eq!(
+            variables.map.get("baibot_model_id"),
+            Some(&"gpt-4o".to_string())
+        );
+        assert_eq!(
+            variables.map.get("baibot_now_utc"),
+            Some(&format_utc_time(now_utc))
+        );
+
+        let prompt = "Hello, I'm {{ baibot_name }} using {{ baibot_model_id }}. The date/time now is {{ baibot_now_utc }}.";
+        let expected = "Hello, I'm baibot using gpt-4o. The date/time now is 2024-09-20 (Friday), 18:34:15 UTC.";
+
+        assert_eq!(variables.format(prompt), expected);
+    }
+}