fix(agent): keep _serialize_for_summary byte-identical to main

The record split added .rstrip("\n") to every serialized record. That
changes the bytes _serialize_for_summary returns for legacy tail_mode
and for agent/micro_compaction.py's _micro_summarize_one, which never
sample and never needed it: a message with trailing newlines was no
longer reproduced verbatim in the summarizer prompt. Sampling receives
records structurally, so an empty pseudo-record can no longer become the
tail anchor and the strip has no remaining purpose.

agent/context_compressor.py

@@ -3320,11 +3320,11 @@ class ContextCompressor(SummaryDispatchMixin, MicroCompactionMixin, ContextEngin
             if len(content) > self._CONTENT_MAX:
                 content = content[:self._CONTENT_HEAD] + "\n...[truncated]...\n" + content[-self._CONTENT_TAIL:]
             if role == "tool":
-                parts.append(f"[TOOL RESULT {msg.get('tool_call_id', '')}]: {content.rstrip(chr(10))}")
+                parts.append(f"[TOOL RESULT {msg.get('tool_call_id', '')}]: {content}")
                 continue
             if role == "assistant" and msg.get("tool_calls", []):
                 content += "\n[Tool calls:\n" + "\n".join(map(self._render_tool_call_for_summary, msg["tool_calls"])) + "\n]"
-            parts.append(f"[{role.upper()}]: {content.rstrip(chr(10))}")
+            parts.append(f"[{role.upper()}]: {content}")
         return parts
 
     def _serialize_for_summary(self, turns: List[Dict[str, Any]]) -> str: