assistant: Add basic tool invocation (#17368)

This PR adds the initial groundwork for invoking tools in response to
tool uses from the model.

Tool uses are run when the model responds with a `stop_reason` of
`tool_use`.

Currently the tool results are just inserted as text into the user
message. We'll want to include these as `tool_result` content on the
message, but Claude seems to understand it regardless.

Release Notes:

- N/A

crates/assistant_tool/src/tool_registry.rs

@@ -66,4 +66,9 @@ impl ToolRegistry {
     pub fn tools(&self) -> Vec<Arc<dyn Tool>> {
         self.state.read().tools.values().cloned().collect()
     }
+
+    /// Returns the [`Tool`] with the given name.
+    pub fn tool(&self, name: &str) -> Option<Arc<dyn Tool>> {
+        self.state.read().tools.get(name).cloned()
+    }
 }