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_models/src/provider/anthropic.rs

@@ -498,7 +498,7 @@ pub fn map_to_language_model_completion_events(
                                     Some(maybe!({
                                         Ok(LanguageModelCompletionEvent::ToolUse(
                                             LanguageModelToolUse {
-                                                id: tool_use.id,
+                                                id: tool_use.id.into(),
                                                 name: tool_use.name,
                                                 input: if tool_use.input_json.is_empty() {
                                                     serde_json::Value::Null