fix: zeroize full string capacity when building shroudedstring from string

[mooncitydev]

Issue

write_and_zeroize_source only cleared len bytes of the source buffer; any spare capacity in the heap-backed String could still retain secret material after the copy.

Fix

Call Zeroize::zeroize on the String after the copy completes so the entire allocation (including slack capacity) is cleared, consistent with the zeroize crate behavior for String.

made by mooncitydev