Apply rate limits in LLM service (#15997)

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/rpc/src/llm.rs

@@ -1,9 +1,13 @@
 use serde::{Deserialize, Serialize};
+use strum::{Display, EnumIter, EnumString};
 
 pub const EXPIRED_LLM_TOKEN_HEADER_NAME: &str = "x-zed-expired-token";
 
-#[derive(Debug, PartialEq, Eq, Clone, Copy, Serialize, Deserialize)]
+#[derive(
+    Debug, PartialEq, Eq, Hash, Clone, Copy, Serialize, Deserialize, EnumString, EnumIter, Display,
+)]
 #[serde(rename_all = "snake_case")]
+#[strum(serialize_all = "snake_case")]
 pub enum LanguageModelProvider {
     Anthropic,
     OpenAi,