refactor: align test style with repo conventions

- Flatten test classes into standalone functions (matching pinecone/qdrant pattern)
- Assert full serialized dict structure in to_dict/from_dict tests
- Use Mock(spec=...) for retriever tests instead of MagicMock+patch
- Verify _embedding_retrieval call args match exactly
- Add test_from_dict_no_filter_policy (backward compat)
- Add test_init_is_lazy

Author: dotKokott

integrations/amazon_s3_vectors/pyproject.toml

@@ -147,7 +147,7 @@ ban-relative-imports = "parents"
 
 [tool.ruff.lint.per-file-ignores]
 # Tests can use magic values, assertions, relative imports, and don't need type annotations
-"tests/**/*" = ["PLC0415", "PLR2004", "S101", "S110", "TID252", "D", "ANN"]
+"tests/**/*" = ["E501", "PLC0415", "PLR2004", "S101", "S110", "TID252", "D", "ANN"]
 # Examples can contain print statements and long lines
 "examples/**/*" = ["D", "T201", "E501"]