fix: apply URL override to cached discovery docs and fix base_url path

Two bugs in the initial implementation:
- Cache path returned early without applying the URL rewrite, so
  requests still went to googleapis.com after the first fetch
- base_url incorrectly prepended service name/version, causing
  doubled path segments (e.g., gmail/v1/gmail/v1/...)

Extract apply_base_url_override() helper and call it in both the
cache and network paths. Set base_url to just the custom host since
method paths already contain the full API prefix.

Author: xdotli

src/discovery.rs

@@ -230,7 +230,8 @@ pub async fn fetch_discovery_document(
             if let Ok(modified) = metadata.modified() {
                 if modified.elapsed().unwrap_or_default() < std::time::Duration::from_secs(86400) {
                     let data = std::fs::read_to_string(&cache_file)?;
-                    let doc: RestDescription = serde_json::from_str(&data)?;
+                    let mut doc: RestDescription = serde_json::from_str(&data)?;
+                    apply_base_url_override(&mut doc);
                     return Ok(doc);
                 }
             }
@@ -264,20 +265,19 @@ pub async fn fetch_discovery_document(
     }
 
     let mut doc: RestDescription = serde_json::from_str(&body)?;
+    apply_base_url_override(&mut doc);
+    Ok(doc)
+}
 
-    // When a custom API base URL is set, rewrite the Discovery Document's
-    // root_url and base_url so that all HTTP requests go to the custom
-    // endpoint (e.g., a mock server) instead of the real Google APIs.
+/// Rewrite Discovery Document URLs when `GWS_API_BASE_URL` is set.
+/// Uses the same base_url structure as the original document — just
+/// swaps the host so request paths stay correct for the mock server.
+fn apply_base_url_override(doc: &mut RestDescription) {
     if let Some(base) = custom_api_base_url() {
         let base_trimmed = base.trim_end_matches('/');
         doc.root_url = format!("{base_trimmed}/");
-        doc.base_url = Some(format!(
-            "{base_trimmed}/{}/{}/",
-            doc.name, doc.version
-        ));
+        doc.base_url = Some(format!("{base_trimmed}/"));
     }
-
-    Ok(doc)
 }
 
 #[cfg(test)]