feat(storage): support copying CSEK-encrypted files to non-CSEK destinations

[thiyaguk09]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request adds support for null encryption keys to allow copying CSEK-encrypted files to standard non-CSEK destinations. The review identifies a critical bug in the copy logic where the source file's encryption key is mutated and the destination file is left in an inconsistent state by bypassing setEncryptionKey. Feedback also suggests correcting the corresponding unit tests to avoid asserting on this buggy behavior and to use the proper setter method instead of manually assigning dummy properties.

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