assistant: Add support for displaying billing-related errors (#19082)

This PR adds support to the assistant for display billing-related
errors.

Pulling this out of #19081 to make it easier to cherry-pick.

Release Notes:

- N/A

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Richard <richard@zed.dev>

crates/rpc/src/llm.rs

@@ -3,6 +3,8 @@ use strum::{Display, EnumIter, EnumString};
 
 pub const EXPIRED_LLM_TOKEN_HEADER_NAME: &str = "x-zed-expired-token";
 
+pub const MAX_LLM_MONTHLY_SPEND_REACHED_HEADER_NAME: &str = "x-zed-llm-max-monthly-spend-reached";
+
 #[derive(
     Debug, PartialEq, Eq, Hash, Clone, Copy, Serialize, Deserialize, EnumString, EnumIter, Display,
 )]