test(vertex_ai): align signature tests with checksummed tool call ids

Rework onto main renamed the helpers to public names and upstream added
factory tests pinning the pre-checksum wire format, so update those
expectations to the checksummed layout and use a valid base64 signature
for the embedded-id round trip.

tests/unit/litellm_core_utils/prompt_templates/test_litellm_core_utils_prompt_templates_factory.py

@@ -1,4 +1,4 @@
-import asyncio, base64, importlib, uuid
+import asyncio, base64, hashlib, importlib, uuid
 import json
 import logging
 import os
@@ -83,7 +83,12 @@ def test_function_call_prompt_lets_the_model_answer_after_a_function_result() ->
 @pytest.mark.parametrize(
     ("thought_signature", "expected"),
     [
-        ("encoded-signature", "call_123__thought__encoded-signature"),
+        (
+            "encoded-signature",
+            "call_123__checksum__"
+            + hashlib.sha256(b"encoded-signature").hexdigest()
+            + "__thought__encoded-signature",
+        ),
         (None, "call_123"),
         ("", "call_123"),
     ],
@@ -101,8 +106,12 @@ def test_encode_tool_call_id_with_signature(thought_signature, expected):
             "function-signature",
         ),
         (
-            {"id": encode_tool_call_id_with_signature("call_123", "embedded-signature")},
-            "embedded-signature",
+            {
+                "id": encode_tool_call_id_with_signature(
+                    "call_123", "ZW1iZWRkZWQtc2lnbmF0dXJl"
+                )
+            },
+            "ZW1iZWRkZWQtc2lnbmF0dXJl",
         ),
         ({}, None),
     ],

tests/unit/llms/vertex_ai/gemini/test_thought_signature_in_tool_call_id.py

@@ -322,13 +322,13 @@ def test_get_thought_signature_rejects_decodable_tampering():
     )
 
     assert _is_valid_thought_signature(decodable_but_wrong) is True
-    assert _get_thought_signature_from_tool({"id": tampered_id, "type": "function"}) is None
+    assert get_thought_signature_from_tool({"id": tampered_id, "type": "function"}) is None
 
 
 def test_get_thought_signature_rejects_unsigned_embedded_signature():
     unsigned_id = f"call_abc123{THOUGHT_SIGNATURE_SEPARATOR}{REAL_SIGNATURE}"
 
-    assert _get_thought_signature_from_tool({"id": unsigned_id, "type": "function"}) is None
+    assert get_thought_signature_from_tool({"id": unsigned_id, "type": "function"}) is None
 
 
 def test_get_thought_signature_drops_client_mangled_id_suffix():