language_models: Send a header indicating that the client supports xAI models (#38931)

This PR adds an `x-zed-client-supports-x-ai` header to the `GET /models`
request sent to Cloud to indicate that the client supports xAI models.

Release Notes:

- N/A

crates/cloud_llm_client/src/cloud_llm_client.rs

@@ -55,6 +55,9 @@ pub const CLIENT_SUPPORTS_STATUS_MESSAGES_HEADER_NAME: &str =
 pub const SERVER_SUPPORTS_STATUS_MESSAGES_HEADER_NAME: &str =
     "x-zed-server-supports-status-messages";
 
+/// The name of the header used by the client to indicate that it supports receiving xAI models.
+pub const CLIENT_SUPPORTS_X_AI_HEADER_NAME: &str = "x-zed-client-supports-x-ai";
+
 #[derive(Debug, PartialEq, Clone, Copy, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case")]
 pub enum UsageLimit {