Add regression test for JS/Python snippet limit parity

Prevents the JS slice(0,N) and Python _TOOL_RESULT_SNIPPET_MAX from
drifting apart again.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_tool_snippet_limit_parity.py

@@ -0,0 +1,11 @@
+"""Regression test: JS tool-result snippet limit matches the Python backend limit."""
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parent.parent
+
+
+def test_tool_snippet_limit_parity():
+    py = (REPO / "api" / "streaming.py").read_text(encoding="utf-8")
+    js = (REPO / "static" / "ui.js").read_text(encoding="utf-8")
+    assert "_TOOL_RESULT_SNIPPET_MAX = 4000" in py
+    assert ".slice(0,4000)" in js