fix(mktemp): preserve .. literal prefix in template (#12312)

[0xSoftBoi]

Summary

mktemp /tmp/..XXXXXX was silently dropping the leading .., producing /tmp/<random> instead of /tmp/..<random>.

# before
$ mktemp /tmp/..XXXXXX
/tmp/fpQ7Y8

# after / GNU mktemp
$ mktemp /tmp/..XXXXXX
/tmp/..cmJm2X

Root cause

The (directory, prefix) split in Params::from relied on Path::file_name() and Path::parent() to break up the joined prefix_path. Path::file_name() returns None for any path whose final component is .. (it is a ParentDir, not a regular name), so the prefix fell back to the empty string while Path::parent() popped the preceding component — together they erased the .. from the output filename.

Fix

Detect a trailing .. component on prefix_from_template and split the directory and prefix from the raw template string before path normalization discards it, mirroring the existing handling for templates ending in MAIN_SEPARATOR. Every other template continues through the unchanged code path. This is intentionally a sibling fix to #12311 (which addresses the lone . case via the same hook).

Tests

Added two regression tests in tests/by-util/test_mktemp.rs:

• test_mktemp_dotdot_prefix — bare <dir>/..XXXXXX template: asserts the resulting file name starts with .., matches the expected length, lives in <dir> (not <dir>/..), and actually exists on disk.
• test_mktemp_dotdot_prefix_with_tmpdir_flag — same property with -p <dir> ..XXXXXX.

Full cargo test --test tests test_mktemp is green on macOS (40 passed, 5 linux-only filtered), and cargo fmt --all -- --check and cargo clippy -p uu_mktemp --all-targets -- -D warnings are clean.

Fixes #12312

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)