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/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,