Raise RateLimitError on CDX API 503 responses

Fixes #5. CdxApi#cdx_get now raises RateLimitError instead of generic
Error when the CDX Server returns HTTP 503, consistent with SaveApi's
rate-limit handling.

Author: ronaldtse

lib/archaeo/cdx_api.rb

@@ -151,6 +151,11 @@ def cdx_get(params)
       )
       return response if response.status == 200
 
+      if response.status == 503
+        raise RateLimitError,
+              "CDX API rate limited (HTTP 503)"
+      end
+
       raise Error, "CDX API returned HTTP #{response.status}"
     end
 

spec/archaeo/cdx_api_spec.rb

@@ -199,6 +199,17 @@ def cdx_json_response(rows, resume_key: nil)
       end
     end
 
+    context "with rate limit (503)" do
+      let(:fake_client) do
+        FakeHttpClient.new([FakeHttpClient.response(status: 503)])
+      end
+
+      it "raises RateLimitError" do
+        expect { client.snapshots("example.com").to_a }
+          .to raise_error(Archaeo::RateLimitError, /rate limited/)
+      end
+    end
+
     context "with invalid match_type" do
       let(:fake_client) { FakeHttpClient.new([]) }