test: add Ci solver pFUnit unit test suite and optional error flag

[johnpaulalex]

(not intending to submit yet, just sending for discussion)

Adds a new leaf_biophysics_solver test suite targeting the intracellular CO2 (Ci) solver stack in biogeophys/LeafBiophysicsMod.F90:

• CiMinMax: verifies bracket validity (ci_min < ci_max, physically bounded) and that fval at each endpoint straddles zero
• CiFunc: verified indirectly via test_CiFunc_zero_at_solution, which asserts |fval| < ci_tol at the Ci returned by CiBisection. Documents the decision process not to use an analytical root due to Medlyn complexity.
• CiBisection: convergence under full light (anet > 0, gs > gs0, solve_iter < 200), darkness (anet < 0, gs ≈ gs0), varying tolerances (loose vs tight), and different environmental temperatures/low-light.
• Max Iterations: test_CiBisection_max_iters_sets_ierr tests the maximum iteration failure path by passing ci_tol=0.

To allow testing the failure path, an optional ierr argument is added to CiBisection in biogeophys/LeafBiophysicsMod.F90. If present, the solver returns gracefully with ierr=1 when max iterations is reached instead of calling endrun(). Traditional callers that omit ierr retain the original behavior.

New files:
testing/tests/unit/leaf_biophysics_solver_test/test_LeafBiophysicsSolvers.pf
testing/tests/unit/leaf_biophysics_solver_test/CMakeLists.txt

Integration:
testing/CMakeLists.txt
testing/config/unit.cfg

Description:

Collaborators:

Expectation of Answer Changes:

Description of generative AI usage (as necessary)

Checklist

If this is your first time contributing, please read the CONTRIBUTING document.

All checklist items must be checked to enable merging this pull request:

Contributor

• The in-code documentation has been updated with descriptive comments
• The documentation has been assessed to determine if updates are necessary
• Describe use of generative AI (if necessary)

Integrator

• FATES PASS/FAIL regression tests were run
• Evaluation of test results for answer changes was performed and results provided
• FATES-CLM6 Code Freeze: satellite phenology regression tests are b4b

If satellite phenology regressions are not b4b, please hold merge and notify the FATES development team.

Documentation

• Technical Note update:
• User's Guide update:

Test Results:

CTSM (or) E3SM (specify which) test hash-tag:

CTSM (or) E3SM (specify which) baseline hash-tag:

FATES baseline hash-tag:

Test Output:

[rgknox]

@johnpaulalex , I was curious, have you triggered any failures in the photosynthesis code with your tests?