assistant2: Restructure storage of tool uses and results (#21194)

This PR restructures the storage of the tool uses and results in
`assistant2` so that they don't live on the individual messages.

It also introduces a `LanguageModelToolUseId` newtype for better type
safety.

Release Notes:

- N/A

crates/language_model/src/language_model.rs

@@ -63,9 +63,27 @@ pub enum StopReason {
     ToolUse,
 }
 
+#[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize)]
+pub struct LanguageModelToolUseId(Arc<str>);
+
+impl fmt::Display for LanguageModelToolUseId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl<T> From<T> for LanguageModelToolUseId
+where
+    T: Into<Arc<str>>,
+{
+    fn from(value: T) -> Self {
+        Self(value.into())
+    }
+}
+
 #[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize)]
 pub struct LanguageModelToolUse {
-    pub id: String,
+    pub id: LanguageModelToolUseId,
     pub name: String,
     pub input: serde_json::Value,
 }

crates/language_model/src/request.rs

@@ -347,7 +347,7 @@ impl LanguageModelRequest {
                             }
                             MessageContent::ToolUse(tool_use) => {
                                 Some(anthropic::RequestContent::ToolUse {
-                                    id: tool_use.id,
+                                    id: tool_use.id.to_string(),
                                     name: tool_use.name,
                                     input: tool_use.input,
                                     cache_control,