Added more expansive feedstock tests for SLC

[johnjasa]

Add tests for multi-feedstock handling in the system-level controller

1. Verified that the existing cost_per_tech: feedstock logic in SystemLevelControlBase already discovers all upstream feedstocks via nx.ancestors (any graph depth) and sums their first-year VarOpEx values when computing the dispatchable technology's marginal cost. No code changes were required.
2. Added three new unit tests in test_slc_controllers.py to lock in this behavior against the fuel-cell scenario in the issue (a dispatchable consuming both hydrogen and oxygen as feedstocks):
   • test_feedstock_fuel_cell_h2_o2 exercises a fuel cell with H2 and O2 feedstocks under CostMinimizationControl and checks that the combined feedstock cost yields full dispatch.
   • test_feedstock_multiple_sum_not_average pins the marginal-cost combination to a sum (not a mean) by selecting VarOpEx values where the summed cost is just above the sell price; averaging would incorrectly dispatch the tech.
   • test_feedstock_indirect_upstream connects one feedstock directly and another through an intermediate combiner, verifying that the ancestor-based discovery sees both.

Section 1: Type of Contribution

• Feature Enhancement
  • Framework
  • New Model
  • Updated Model
  • Tools/Utilities
  • Other (please describe):
• Bug Fix
• Documentation Update
• CI Changes
• Other (please describe): test coverage additions

Section 2: Draft PR Checklist

• Open draft PR
• Describe the feature that will be added
• Fill out TODO list steps
• Describe requested feedback from reviewers on draft PR
• Complete Section 8: New Model Checklist (if applicable)

Type of Reviewer Feedback Requested (on Draft PR)

Structural feedback:

• N/A

Implementation feedback:

• N/A

Other feedback:

• N/A

Section 3: General PR Checklist

• PR description thoroughly describes the new feature, bug fix, etc.
• Added tests for new functionality or bug fixes
• Tests pass (If not, and this is expected, please elaborate in the Section 6: Test Results)
• Documentation
  • Docstrings are up-to-date
  • Related docs/ files are up-to-date, or added when necessary
  • Documentation has been rebuilt successfully
  • Examples have been updated (if applicable)
• CHANGELOG.md
  • At least one complete sentence has been provided to describe the changes made in this PR
  • After the above, a hyperlink has been provided to the PR using the following format:
    "A complete thought. PR XYZ", where
    XYZ should be replaced with the actual number.

Section 4: Related Issues

Fully resolves #789

Section 5: Impacted Areas of the Software

Section 5.1: New Files

• N/A

Section 5.2: Modified Files

• h2integrate/control/control_strategies/system_level/test/test_slc_controllers.py - adds three multi-feedstock tests covering the fuel cell H2+O2 case, sum-vs-average correctness, and indirect graph ancestry.

Section 6: Additional Supporting Information

The existing test_feedstock_multiple already covered a 2-feedstock summation case with generic names, but we now cover the fuel-cell H2+O2 scenario plus assurance that the summation is correct under conditions where averaging would change the outcome. The new tests address both.

Section 7: Test Results, if applicable

$ python -m pytest h2integrate/control/control_strategies/system_level/test/test_slc_controllers.py -q
28 passed in 0.58s

Section 8 (Optional): New Model Checklist

N/A