Make plans backwards compatible (#37941)

This PR fixes the backwards compatibility of the new `Plan` variants.

We can't add new variants to the wire representation, as old clients
won't be able to understand them.

Release Notes:

- N/A

crates/agent_ui/src/message_editor.rs

@@ -17,7 +17,7 @@ use agent::{
 use agent_settings::{AgentProfileId, AgentSettings, CompletionMode};
 use ai_onboarding::ApiKeysWithProviders;
 use buffer_diff::BufferDiff;
-use cloud_llm_client::CompletionIntent;
+use cloud_llm_client::{CompletionIntent, PlanV1};
 use collections::{HashMap, HashSet};
 use editor::actions::{MoveUp, Paste};
 use editor::display_map::CreaseId;
@@ -1298,7 +1298,9 @@ impl MessageEditor {
             return None;
         }
 
-        let plan = user_store.plan().unwrap_or(cloud_llm_client::Plan::ZedFree);
+        let plan = user_store
+            .plan()
+            .unwrap_or(cloud_llm_client::Plan::V1(PlanV1::ZedFree));
 
         let usage = user_store.model_request_usage()?;