Move report_assistant_event into language_model crate (#25508)

This PR moves the `report_assistant_event` function from the
`language_models` crate to the `language_model` crate.

This allows us to drop some dependencies on `language_models`.

Release Notes:

- N/A

crates/assistant2/src/buffer_codegen.rs

@@ -9,10 +9,9 @@ use futures::{channel::mpsc, future::LocalBoxFuture, join, SinkExt, Stream, Stre
 use gpui::{App, AppContext as _, Context, Entity, EventEmitter, Subscription, Task};
 use language::{line_diff, Buffer, IndentKind, Point, TransactionId};
 use language_model::{
-    LanguageModel, LanguageModelRegistry, LanguageModelRequest, LanguageModelRequestMessage,
-    LanguageModelTextStream, Role,
+    report_assistant_event, LanguageModel, LanguageModelRegistry, LanguageModelRequest,
+    LanguageModelRequestMessage, LanguageModelTextStream, Role,
 };
-use language_models::report_assistant_event;
 use multi_buffer::MultiBufferRow;
 use parking_lot::Mutex;
 use prompt_library::PromptBuilder;