Updates for AIGFS work

[maddenp-cu]

Synopsis

Working on AIGFS, I noticed a possibility for three improvements:

1. When calling uwtools.api.utils.run_shell_cmd(), the directory specified by cwd must already exist. As of this PR, and as in general in uwtools, the required directory is created if it does not already exist.
2. Also in uwtools.api.utils.run_shell_cmd(), when taskname is provided, it was being prepended for only some of the log messages. This PR corrects that. See below for example bad output.
3. The atomic() context manager is borrowed from wxvx, added to uwtools.utils.file, and exposed via the API. See the corresponding unit test for an example use case.

Here's some output from some AIGFS testing I was doing:

[2026-07-29T04:38:20]     INFO [GRIB index /tmp/aigfs/out/fstab.idx] Running: wgrib2 -s /etc/fstab >/tmp/aigfs/out/fstab.idx.tmp && mv /tmp/aigfs/out/fstab.idx.tmp /tmp/aigfs/out/fstab.idx
[2026-07-29T04:38:20]     INFO   in directory
[2026-07-29T04:38:20]     INFO     /tmp/aigfs/out
[2026-07-29T04:38:20]    ERROR [GRIB index /tmp/aigfs/out/fstab.idx] Failed with status: 127
[2026-07-29T04:38:20]    ERROR [GRIB index /tmp/aigfs/out/fstab.idx] Output:
[2026-07-29T04:38:20]    ERROR [GRIB index /tmp/aigfs/out/fstab.idx]   wgrib2: symbol lookup error: wgrib2: undefined symbol: g2c_enc_aec

Never mind the error, but note that the second and third log messages anomalously are not prefixed with the taskname.

Type

• Bug fix (corrects a known issue)
• Enhancement (adds new functionality)

Impact

• This is a non-breaking change (existing functionality continues to work as expected)

Checklist

• I have added myself and any co-authors to the PR's Assignees list.
• I have reviewed the documentation and have made any updates necessitated by this change.
• Where helpful, I have written comments in this PR's Files changed view to assist reviewers.