fix: address coderabbit review feedback

- Add try/except to _build_context, return None when hash fails
- Return None from _serialize_cache_key on total failure (no id()-based fallback)
- Replace hex-like test literal with non-secret placeholder

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests-unit/execution_test/test_cache_provider.py

@@ -333,13 +333,13 @@ class TestCacheContext:
             prompt_id="prompt-123",
             node_id="node-456",
             class_type="KSampler",
-            cache_key_hash="abcdef1234567890" * 4,
+            cache_key_hash="a" * 64,
         )
 
         assert context.prompt_id == "prompt-123"
         assert context.node_id == "node-456"
         assert context.class_type == "KSampler"
-        assert context.cache_key_hash == "abcdef1234567890" * 4
+        assert context.cache_key_hash == "a" * 64
 
 
 class TestCacheValue: