Validate keyfile inputs before volume creation

[srichs]

Motivation

• Surface invalid keyfiles inputs early and prevent malformed or nonexistent keyfile paths from being passed to the VeraCrypt CLI.

Description

• Add a new _validate_keyfiles(self, keyfiles, context) method that ensures keyfiles is a list of strings and that each path exists, raising ValueError or VeraCryptError on problems.
• Invoke _validate_keyfiles(keyfiles, "create_volume") at the start of create_volume to validate keyfile inputs before building CLI commands.

Testing

• No automated tests were executed for this change.

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Codex Task