migration: preserve mentor and llm streaming slice

app/backend/llm/stream.py

@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+from typing import Any
+
+
+class OpenAIStreamAccumulator:
+    """Normalize incremental deltas and provider-specific full-message snapshots."""
+
+    def __init__(self) -> None:
+        self.text = ""
+        self.saw_delta = False
+
+    def feed(self, choice: dict[str, Any]) -> str:
+        delta = choice.get("delta")
+        if isinstance(delta, dict) and delta.get("content") is not None:
+            chunk = str(delta.get("content") or "")
+            if chunk:
+                self.saw_delta = True
+                self.text += chunk
+            return chunk
+
+        message = choice.get("message")
+        if not isinstance(message, dict) or message.get("content") is None:
+            return ""
+        snapshot = str(message.get("content") or "")
+        if not snapshot:
+            return ""
+        if not self.text:
+            self.text = snapshot
+            return snapshot
+        if snapshot == self.text or self.text.startswith(snapshot):
+            return ""
+        if snapshot.startswith(self.text):
+            suffix = snapshot[len(self.text):]
+            self.text = snapshot
+            return suffix
+        if self.saw_delta:
+            # A final full snapshot cannot safely replace chunks already delivered.
+            return ""
+        return ""