Refactor AmeriFlux CF coverage checks into reusable helper

[Akash-paluvai]

Description

This PR extracts the AmeriFlux CF data coverage checks (radiation and soil moisture)
from AmeriFlux_met_ensemble() into a dedicated helper function.

The new helper centralizes logic for:

• detecting variable presence
• computing coverage fractions
• determining which ERA5 variables are required for fallback

No functional behavior is changed; this is a pure refactor to reduce duplication
and improve readability and maintainability.

Motivation and Context

This change is a first, intentionally limited refactor step toward the modular
AmeriFlux workflow described in #3605, following suggestions from #3586.

By isolating coverage checks into a reusable helper, subsequent work (e.g.
metgapfill fallback wiring and met.process() integration) can build on a
well-defined and testable component without re-implementing logic.

Related issue:

• Compose AmeriFlux met ensemble from modular PEcAn steps with ERA5/ERA5-Land fallback #3605

Review Time Estimate

• Immediately
• Within one week
• When possible

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking refactor introducing a reusable helper)
• Breaking change

Checklist:

• My change requires a change to the documentation.
• My name is in the list of CITATION.cff
• I agree that PEcAn Project may distribute my contribution under any or all of
  • the same license as the existing code,
  • and/or the BSD 3-clause license.
• I have updated the CHANGELOG.md.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[infotroph]

Thanks for adding the on.exit -- this is a nice upgrade!!