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/agent/src/buffer_codegen.rs

@@ -425,6 +425,8 @@ impl CodegenAlternative {
         request_message.content.push(prompt.into());
 
         Ok(LanguageModelRequest {
+            thread_id: None,
+            prompt_id: None,
             tools: Vec::new(),
             stop: Vec::new(),
             temperature: None,