test: add the local_beta_headers_config fixture the safeguards tests use

Hand-ported to stable/1.100.x from 47b2479c94 on main (fix(bedrock): gate Invoke tool search on the model map's supports_tool_search flag), the one prerequisite the #42288 handler tests need; the rest of that commit stays on main.

tests/test_litellm/conftest.py

@@ -203,6 +203,21 @@ def local_model_cost_map(monkeypatch):
         litellm.get_model_info.cache_clear()
 
 
+@pytest.fixture
+def local_beta_headers_config(monkeypatch):
+    """Pin the bundled ``anthropic_beta_headers_config.json`` so beta header assertions
+    do not depend on the network-fetched copy or on what earlier tests left cached."""
+    from litellm.anthropic_beta_headers_manager import reload_beta_headers_config
+
+    monkeypatch.setenv("LITELLM_LOCAL_ANTHROPIC_BETA_HEADERS", "True")
+    reload_beta_headers_config()
+    try:
+        yield
+    finally:
+        monkeypatch.delenv("LITELLM_LOCAL_ANTHROPIC_BETA_HEADERS", raising=False)
+        reload_beta_headers_config()
+
+
 def _run_coroutine_if_needed(result):
     if not asyncio.iscoroutine(result):
         return