fix(hydraulics): fix SumBetweenDepths layer integration logic and add pFUnit test

[johnpaulalex]

Description of changes

Fixes partial-layer depth integration calculations in SumBetweenDepths within biogeophys/FatesPlantHydraulicsMod.F90. Specifically:

1. Single-layer integration fix: Handles cases where both the top depth (depth_t) and bottom depth (depth_b) fall within the exact same soil rhizosphere layer (i_rhiz_t - 1 == i_rhiz_b + 1). Previously, partial layer fractions were miscalculated or double-counted across layer boundaries.
2. Layer 0 boundary fix: Adds explicit boundary handling for i_rhiz_b == 0 when integrating from the surface down through the top layer (frac = depth_b / dz_rhiz(1)).
3. pFUnit Test Suite: Exposes SumBetweenDepths for unit testing and adds comprehensive test coverage (test_SumBetweenDepths in FatesPlantHydraulicsMod_geom_test.pf) verifying integration density across non-uniform soil layer interfaces, partial overlaps, and out-of-bound depth limits.

Specific notes

Contributors other than yourself, if any:

• @jpalex

Linked issues addressed, if any:

• Contributes to Add summary metrics for btran (and expand capability of running averages) NGEET/fates#1409

Description of generative AI usage:

• Google Antigravity was used to write the code and tests, followed by human-guided verification.

Answer Changes & Scientific Impact

• Expected Answer Changes (ECA)
  • Details: Answers change for simulations with active plant hydraulics when depth integration intervals fall within a single soil layer or at the topmost layer boundary (i_rhiz_b == 0). Simulations without plant hydraulics remain Bit-for-Bit (B4B).

User Interface & Namelist Changes

• Namelist / Defaults modified? No.
• XML / Build script changes? Updated testing/CMakeLists.txt and testing/config/unit.cfg to register fates_plant_hydraulics_test.

Testing planned or performed, if any:

• Executed pFUnit unit tests for FatesPlantHydraulicsMod:
  • FatesPlantHydraulicsMod_cohort_test
  • FatesPlantHydraulicsMod_drive_test
  • FatesPlantHydraulicsMod_geom_test
  • FatesPlantHydraulicsMod_sizedep_test
• Verified clean compilation on gfortran/Intel compilers.

FATES baseline hash-tag: 5efa6fbf
PR branch hash-tag: de7afd15

Requirements before merge:

• The code in this PR branch builds with no errors.
• The code in this PR branch runs with no errors.
• In-code documentation and Fortran docstrings updated.
• This PR either (a) does not create a need to update documentation or (b) includes required documentation updates. Which?: (a) Bug fix and unit test addition; no user guide or tech note changes required.

[johnpaulalex]

Closing draft PR on fork; moved to draft PR on main repository: NGEET#1603