refactor: manage password handling in Excel processing

[psxjoy]

Purpose of the pull request

Improve the lifecycle handling of POI encryption context in XLS read/write paths.

What's changed?

• Scope encryption context setup to the XLS operations that require it.
• Clear the context in finally after XLS sheet listing, parsing, and write protection.
• Add regression assertions to verify the context is cleared after encrypted read/write flows.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[pjfanning]

lgtm