fix(utils): share the Saver frequency decision across models in a step

[MohammadHijjawi97]

Description

PPOTrainer._save_hf calls Saver.save for the actor and then for the critic in the same training step. Each call ran the shared EpochStepTimeFreqCtl.check, so the step and epoch counters advanced twice per step, and the time window was reset by the first call. As a result, when a critic is used:

• saver.freq_steps=2 (or freq_epochs=2): the critic is saved every step (or epoch) and the actor is never saved.
• saver.freq_secs=N: the first call resets the window, so the actor is saved and the critic is not.

This PR makes the frequency decision once per global step in Saver and reuses it for the other models saved in that step. Trainers that save a single model behave exactly as before, and the saved freq_ctl state format is unchanged.

Reproduction (freq_steps=2, actor + critic saved each step, 6 steps):

before: actor saves at steps: []               critic saves at steps: ['0', '1', '2', '3', '4', '5']
after:  actor saves at steps: ['1', '3', '5']  critic saves at steps: ['1', '3', '5']

Related Issue

None; no existing issue or PR covers it.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation update
• ♻️ Refactoring
• ⚡ Performance improvement
• ✅ Test coverage improvement

Checklist

• I have read the Contributing Guide
• Pre-commit hooks pass (pre-commit run --all-files)
• Relevant tests pass; new tests added for new functionality
• Documentation updated (if applicable; built with ./docs/build_all.sh)
• Branch is up to date with main
• Self-reviewed via /review-pr command
• This PR was created by a coding agent via /create-pr
• This PR is a breaking change

Additional Context

Added test_save_applies_frequency_once_per_step_for_all_models to tests/test_saver.py, parametrized over freq_steps=2 and freq_epochs=2. It fails on main (the actor is never saved) and passes with this change. All of tests/test_saver.py passes (6 tests). pre-commit run --files on the two changed files passes (ruff lint/format, SPDX header). These are CPU-only unit tests; I did not run a full PPO training job.