fix: never leave plaintext when encryption fails

[nkuprins]

Closed: #1135

Purpose of the pull request

As title
Related: #1168 fixes the separate temp-file leak on the same path (#1137).

What's changed?

• OutputStream: when doOutputStreamEncrypt07() fails, finish() sets writeExcel = false, so the unencrypted workbook is no longer written to the caller's stream.
• File: when doFileEncrypt07() fails, finish() deletes the target file.
• If deleting the target file fails, the reported error is "Can not delete unencrypted file: ", with the encryption error as its cause. It is recorded like the other failures in finish() rather than thrown on the spot, so the remaining cleanup still runs.

Tests

• xlsxStreamPasswordWrite_encryptionFails_writesNothing: the temp directory path runs through a regular file, so it cannot be created. Asserts the caller's stream is empty; on main it holds the full unencrypted workbook.
• xlsxFilePasswordWrite_encryptionFails_leavesNoFile: the target is made read-only after its output stream is open, so encrypting it in place fails. Asserts the file no longer exists; on main it is left as a plain xlsx. Skipped where file permissions are not enforced (root).

This conflicts with #1168 only at the end of EncryptDataTest, where both add tests. I'll rebase whichever lands second; after #1168 is merged, the stream test here can use its writeWithPassword helper.

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.