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/prompt_library/src/prompt_library.rs

@@ -924,6 +924,8 @@ impl PromptLibrary {
                         .update(|_, cx| {
                             model.count_tokens(
                                 LanguageModelRequest {
+                                    thread_id: None,
+                                    prompt_id: None,
                                     messages: vec![LanguageModelRequestMessage {
                                         role: Role::System,
                                         content: vec![body.to_string().into()],