fix: use hashable types in frozenset test and add dict test

Frozensets can only contain hashable types, so use nested frozensets
instead of dicts. Added separate test for dict handling via serialize_cache_key.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests-unit/execution_test/test_cache_provider.py

@@ -142,10 +142,12 @@ class TestSerializeCacheKey:
 
     def test_complex_nested_structure(self):
         """Complex nested structures should hash deterministically."""
+        # Note: frozensets can only contain hashable types, so we use
+        # nested frozensets of tuples to represent dict-like structures
         key = frozenset([
             ("node_1", frozenset([
                 ("input_a", ("tuple", "value")),
-                ("input_b", {"nested": "dict"}),
+                ("input_b", frozenset([("nested", "dict")])),
             ])),
             ("node_2", frozenset([
                 ("param", 42),
@@ -158,6 +160,18 @@ class TestSerializeCacheKey:
 
         assert hash1 == hash2
 
+    def test_dict_in_cache_key(self):
+        """Dicts passed directly to serialize_cache_key should work."""
+        # This tests the _canonicalize function's ability to handle dicts
+        key = {"node_1": {"input": "value"}, "node_2": 42}
+
+        hash1 = serialize_cache_key(key)
+        hash2 = serialize_cache_key(key)
+
+        assert hash1 == hash2
+        assert isinstance(hash1, bytes)
+        assert len(hash1) == 32
+
 
 class TestContainsNan:
     """Test contains_nan utility function."""