fix: use APIStatusError.for instead of APIError.for in stream.rb (#194)

The APIError class doesn't have a 'for' class method, but APIStatusError does.
This fixes a NoMethodError that would occur when streaming encounters an error.

test: add test for streaming error handling with APIStatusError.for

This test verifies that streaming error responses properly use
APIStatusError.for instead of the non-existent APIError.for method.

Author: samuelreh

test/openai/resources/responses/streaming_test.rb

@@ -212,6 +212,31 @@ def test_incomplete_streaming
     end
   end
 
+  def test_streaming_error_handling
+    stub_request(:post, "http://localhost/responses")
+      .with(
+        body: hash_including(
+          instructions: "You are a helpful assistant",
+          messages: [{content: "Hello", role: "user"}],
+          model: "gpt-4",
+          stream: true
+        )
+      )
+      .to_return(
+        status: 400,
+        headers: {"Content-Type" => "text/event-stream"},
+        body: error_sse_response
+      )
+
+    stream = @client.responses.stream(**basic_params)
+    error = assert_raises(OpenAI::Errors::APIStatusError) do
+      stream.each { |_event| } # Consume the stream to trigger the error.
+    end
+
+    assert_equal(400, error.status)
+    assert_equal("Invalid request: missing required field", error.message)
+  end
+
   def test_text_method
     stub_streaming_response(basic_text_sse_response)
 
@@ -686,4 +711,14 @@ def resume_stream_structured_output_sse_response
 
     SSE
   end
+
+  def error_sse_response
+    <<~SSE
+      event: response.created
+      data: {"type":"response.created","sequence_number":1,"response":{"id":"msg_error","object":"realtime.response","status":"in_progress","status_details":null,"output":[],"usage":null,"metadata":null}}
+
+      data: {"error":{"message":"Invalid request: missing required field"}}
+
+    SSE
+  end
 end