agent: Show an error when the model requests limit has been reached (#28868)

This PR adds an error message when the model requests limit has been
hit.

Release Notes:

- N/A

Co-authored-by: Oleksiy Syvokon <oleksiy.syvokon@gmail.com>

crates/language_models/Cargo.toml

@@ -53,6 +53,7 @@ tokio = { workspace = true, features = ["rt", "rt-multi-thread"] }
 ui.workspace = true
 util.workspace = true
 workspace-hack.workspace = true
+zed_llm_client.workspace = true
 
 [dev-dependencies]
 editor = { workspace = true, features = ["test-support"] }