fix: consistent directory creation message for 7z

[chiliec]

Closes #328

zip and tar already log Directory "..." created for directory entries, but 7z still printed the old File <name> extracted to "..." line:

[INFO] File input extracted to "out/input"
[INFO] File input/sub extracted to "out/input/sub"

This switches the 7z branch to the same message, so all three archive formats produce identical output for directories.

Added ui_test_ok_decompress_multiple_files_7z, a 7z counterpart of the existing ui_test_ok_decompress_multiple_files snapshot test (same expected output as the tar.zst snapshot). It fails before the change and passes after.

Validation (local, Linux x86_64, toolchain 1.93.0):

• cargo test --no-default-features --features unrar,use_zlib,use_zstd_thin: all suites pass (53 + 3 + 57 + 3 + 14 + 2)
• cargo fmt --check: clean

I couldn't build the default bzip3 feature locally (bindgen needs libclang, which isn't on this machine); the change doesn't touch bzip3.