assistant: Require user to accept TOS for cloud provider (#16111)

This adds the requirement for users to accept the terms of service the
first time they send a message with the Cloud provider.

Once this is out and in a nightly, we need to add the check to the
server side too, to authenticate access to the models.

Demo:


https://github.com/user-attachments/assets/0edebf74-8120-4fa2-b801-bb76f04e8a17



Release Notes:

- N/A

crates/proto/src/proto.rs

@@ -187,6 +187,8 @@ impl fmt::Display for PeerId {
 }
 
 messages!(
+    (AcceptTermsOfService, Foreground),
+    (AcceptTermsOfServiceResponse, Foreground),
     (Ack, Foreground),
     (AckBufferOperation, Background),
     (AckChannelMessage, Background),
@@ -409,6 +411,7 @@ messages!(
 );
 
 request_messages!(
+    (AcceptTermsOfService, AcceptTermsOfServiceResponse),
     (ApplyCodeAction, ApplyCodeActionResponse),
     (
         ApplyCompletionAdditionalEdits,