fix: omit empty scheduler resource directives

[njzjz-bot]

Summary

• omit empty queue directives for PBS, Torque, LSF, JH_UniScheduler, and Fugaku
• omit zero-GPU directives for LSF and JH_UniScheduler
• preserve explicit custom GPU lines even when the numeric GPU count is zero
• add scheduler-header regression coverage for default CPU resources

Closes #611

Tests

• python -m coverage run -p --source=./dpdispatcher -m unittest -v (166 passed, 42 skipped)
• ruff check .
• ruff format --check .
• pyright dpdispatcher/machines/lsf.py dpdispatcher/machines/JH_UniScheduler.py dpdispatcher/machines/fugaku.py dpdispatcher/machines/pbs.py
• dpdisp --help

Coding agent: Codex
Codex version: codex-cli 0.149.0
Model: gpt-5.6-sol
Reasoning effort: xhigh

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📥 Commits

Reviewing files that changed from the base of the PR and between 34ddb4b and 6d73ea1.

📒 Files selected for processing (5)

• dpdispatcher/machines/JH_UniScheduler.py
• dpdispatcher/machines/fugaku.py
• dpdispatcher/machines/lsf.py
• dpdispatcher/machines/pbs.py
• tests/test_optional_scheduler_directives.py

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.80%. Comparing base (f547911) to head (6d73ea1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #636      +/-   ##
==========================================
+ Coverage   48.38%   52.80%   +4.41%     
==========================================
  Files          40       40              
  Lines        3960     3962       +2     
==========================================
+ Hits         1916     2092     +176     
+ Misses       2044     1870     -174     

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