zeta: Restore ZED_PREDICT_EDITS_URL environment variable (#30418)

This PR restores the `ZED_PREDICT_EDITS_URL` that was removed in
https://github.com/zed-industries/zed/pull/30290.

While we don't need to use it anymore for local development against the
LLM Worker, some folks reported using it to run versions of Zeta hosted
elsewhere.

Since we don't yet have an officially-supported mechanism today for
bringing your own Zeta for edit predictions, I'm putting the environment
variable back to not break that use case.

Closes https://github.com/zed-industries/zed/issues/30308.

Release Notes:

- N/A

crates/zeta/src/zeta.rs

@@ -740,13 +740,18 @@ and then another
             let mut did_retry = false;
 
             loop {
-                let request = http_client::Request::builder()
-                    .method(Method::POST)
-                    .uri(
-                        http_client
-                            .build_zed_llm_url("/predict_edits/v2", &[])?
-                            .as_ref(),
-                    )
+                let request_builder = http_client::Request::builder().method(Method::POST);
+                let request_builder =
+                    if let Ok(predict_edits_url) = std::env::var("ZED_PREDICT_EDITS_URL") {
+                        request_builder.uri(predict_edits_url)
+                    } else {
+                        request_builder.uri(
+                            http_client
+                                .build_zed_llm_url("/predict_edits/v2", &[])?
+                                .as_ref(),
+                        )
+                    };
+                let request = request_builder
                     .header("Content-Type", "application/json")
                     .header("Authorization", format!("Bearer {}", token))
                     .header(ZED_VERSION_HEADER_NAME, app_version.to_string())