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/language_model/src/request.rs

@@ -238,6 +238,8 @@ pub struct LanguageModelRequestTool {
 
 #[derive(Clone, Debug, Default, Serialize, Deserialize, PartialEq)]
 pub struct LanguageModelRequest {
+    pub thread_id: Option<String>,
+    pub prompt_id: Option<String>,
     pub messages: Vec<LanguageModelRequestMessage>,
     pub tools: Vec<LanguageModelRequestTool>,
     pub stop: Vec<String>,