fix: initialize optional Amazon S3 parameters

[sdojjy]

Summary

• initialize all optional Amazon S3 storage parameters
• prevent COPY INTO compilation from raising AttributeError when parameters are omitted or set to false
• add regression coverage for omitted and partial optional parameter combinations

Testing

• python -m compileall -q tidbcloudlake_sqlalchemy tests/test_copy_into.py
• standalone SQLAlchemy compilation checks passed for both regression scenarios
• full pytest was not runnable locally because the SQLAlchemy test plugin requires a service at localhost:8000

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