feat(seer): Add preamble to structured page context markdown

Prepend a blockquote explaining that the content is a structured
summary, not an exact screenshot. This helps the LLM understand
the context type and also serves as a marker to differentiate
structured context from raw ASCII screenshots downstream.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Mihir-Mavalankar

src/sentry/seer/explorer/client_utils.py

@@ -369,4 +369,7 @@ def snapshot_to_markdown(snapshot: dict[str, Any]) -> str:
     nodes = snapshot.get("nodes", [])
     if not nodes:
         return ""
-    return _render_node(nodes[0], 0)
+    preamble = (
+        "> This is a structured summary of the page the user is viewing, not an exact screenshot.\n"
+    )
+    return preamble + _render_node(nodes[0], 0)

tests/sentry/seer/explorer/test_client_utils.py

@@ -242,7 +242,8 @@ def test_node_with_no_data(self) -> None:
             "nodes": [{"nodeType": "dashboard", "data": None, "children": []}],
         }
         result = snapshot_to_markdown(snapshot)
-        assert result == "# dashboard"
+        assert "# dashboard" in result
+        assert "not an exact screenshot" in result
 
     def test_node_with_non_dict_data(self) -> None:
         snapshot = {