feat(flux): convert walltime to minutes/FSD for batch_system="flux"

[jasonb5]

Summary

• Adds a flux-specific walltime conversion in EnvBatch.set_job_defaults(): Flux's -t/--time-limit accepts minutes or Flux Standard Duration (FSD, RFC 23), not HH:MM:SS
• On the flux path, %H/%M/%S(/%D) in walltime_format expand to the total duration in that unit, so a matching FSD suffix yields a valid value (e.g. %Mm on 01:10:00 → 70m, %Hh → 1.17h); no format defaults to FSD minutes (70m)
• Accepted formats are whitelisted (%M, %Ss, %Mm, %Hh, %Dd); anything else (e.g. %H — a bare number flux would misread as minutes — or %H:%M:%S) fails loudly at case setup instead of submitting a wrong time limit
• Partial values round up at two decimal places so a job is never allotted less time than requested
• The generic format_time()/walltime_format path for all other batch systems is untouched
• Existing deployed flux config (<walltime_format>%Mm</walltime_format> on Tuolumne) works unchanged

Validation

• New parametrized unit tests for _flux_walltime covering all accepted formats, rounding edge cases, and rejected formats, plus mock-based set_job_defaults tests for the flux branch (01:10:00 → 70m/70/1.17h, 00:10:30 → 11m)
• All existing test_unit_xml_env_batch.py tests pass, confirming non-flux behavior is unchanged
• Docs build cleanly; new "Walltime Format" section documents both the positional and flux semantics
• End-to-end validation on Tuolumne still to be confirmed by a maintainer with access

Closes #5017

[github-actions]

You can preview documentation at https://esmci.github.io/cime/branch/fix/flux-walltime-minutes/html/index.html

[Copilot]

Pull request overview

Adds a Flux-specific walltime rendering path so CIME emits a Flux-compatible --time-limit value (minutes or Flux Standard Duration) without changing the existing format_time() / positional walltime_format behavior for other batch systems. Also updates documentation and adds unit tests to validate conversion behavior and error handling.

Changes:

• Add EnvBatch._flux_walltime() and use it from EnvBatch.set_job_defaults() when batch_system is flux.
• Document Flux-specific walltime_format semantics and accepted formats.
• Add unit tests for Flux walltime conversion and the set_job_defaults() Flux branch.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
doc/source/ccs/model-configuration/variables/batch.rst	Documents walltime_format, including Flux-specific total-duration semantics and accepted formats.
CIME/XML/env_batch.py	Implements Flux-only walltime conversion/validation during job default setup.
CIME/tests/test_unit_xml_env_batch.py	Adds pytest coverage for _flux_walltime() and Flux behavior in set_job_defaults().

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[jasonb5]

Needs testing on actual machine still.