test(streaming): assert interim prose finalizes once

Pin the exact parser finalization count in the interim-assistant-to-tool regression.

Assisted-by: Hermes Agent:gpt-5.6-sol
Assisted-by: Codex:gpt-5.6-luna

tests/test_anchor_prose_incremental_finalize.py

@@ -463,3 +463,4 @@ def test_interim_assistant_to_tool_boundary_must_render_full_authoritative_punct
     assert data["toolSourceEventType"] == "tool"
     assert data["proseSourceText"] == data["authoritativeText"]
     assert data["proseRenderedText"] == data["authoritativeText"]
+    assert data["finalizeCount"] == 1