port ai crate to ai2, with all tests passing

crates/ai2/src/models.rs

@@ -0,0 +1,16 @@
+pub enum TruncationDirection {
+    Start,
+    End,
+}
+
+pub trait LanguageModel {
+    fn name(&self) -> String;
+    fn count_tokens(&self, content: &str) -> anyhow::Result<usize>;
+    fn truncate(
+        &self,
+        content: &str,
+        length: usize,
+        direction: TruncationDirection,
+    ) -> anyhow::Result<String>;
+    fn capacity(&self) -> anyhow::Result<usize>;
+}