agent: Attach thread ID and prompt ID to telemetry events (#29069)

This PR attaches the thread ID and the new prompt ID to telemetry events
for completions in the Agent panel.

Release Notes:

- N/A

---------

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/assistant_context_editor/src/context.rs

@@ -2555,6 +2555,8 @@ impl AssistantContext {
         }
 
         let mut completion_request = LanguageModelRequest {
+            thread_id: None,
+            prompt_id: None,
             messages: Vec::new(),
             tools: Vec::new(),
             stop: Vec::new(),