fix: preserve refresh token when server omits it from refresh response

.github/workflows/test.yml

@@ -0,0 +1,21 @@
+name: Test
+
+on:
+  push:
+    branches: [master, main]
+  pull_request:
+    branches: [master, main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+
+      - run: bun install
+
+      - run: bun test

index.mjs

@@ -129,7 +129,7 @@ export async function AnthropicAuthPlugin({ client }) {
                   },
                   body: {
                     type: "oauth",
-                    refresh: json.refresh_token,
+                    refresh: json.refresh_token ?? auth.refresh,
                     access: json.access_token,
                     expires: Date.now() + json.expires_in * 1000,
                   },