fix: prevent duplicate LLM stream snapshots

tests/test_llm_stream.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+import unittest
+
+from llm_stream import OpenAIStreamAccumulator
+
+
+class OpenAIStreamAccumulatorTests(unittest.TestCase):
+    def test_repeated_delta_chunks_are_preserved_as_model_output(self) -> None:
+        accumulator = OpenAIStreamAccumulator()
+        self.assertEqual(accumulator.feed({"delta": {"content": "yes"}}), "yes")
+        self.assertEqual(accumulator.feed({"delta": {"content": "yes"}}), "yes")
+        self.assertEqual(accumulator.text, "yesyes")
+
+    def test_final_snapshot_can_add_a_missing_suffix(self) -> None:
+        accumulator = OpenAIStreamAccumulator()
+        self.assertEqual(accumulator.feed({"delta": {"content": "first"}}), "first")
+        self.assertEqual(
+            accumulator.feed({"message": {"content": "first second"}}),
+            " second",
+        )
+        self.assertEqual(accumulator.text, "first second")
+
+    def test_incompatible_final_snapshot_is_not_appended_twice(self) -> None:
+        accumulator = OpenAIStreamAccumulator()
+        accumulator.feed({"delta": {"content": "streamed answer"}})
+        self.assertEqual(
+            accumulator.feed({"message": {"content": "rewritten final answer"}}),
+            "",
+        )
+        self.assertEqual(accumulator.text, "streamed answer")
+
+
+if __name__ == "__main__":
+    unittest.main()