test(hydraulics): implement plant hydraulics unit test suite

[johnpaulalex]

Description:

Adds pFUnit unit test suites (cohort, drive, geom, init, sizedep) for FatesPlantHydraulicsMod under testing/tests/unit/fates_plant_hydraulics_test/.

Includes continuous parameter sweeps (test_UpdatePlantPsiFTCFromTheta_sweep) that evaluate relative water content ($\theta$) across a 10-point gradient down to near-residual, verifying matric potential ($\Psi$) and fractional loss of conductivity ($) against independent Van Genuchten and Weibull theoretical oracles.

Parent PR:

• Parent PR: #1599 (Base branch: test/hydr-base)

Preview of Child Branches / Stacked PRs:

• #1603 (fix/sum-btwn-depths+hydr-plant): Fixes SumBetweenDepths soil layer integration logic in FatesPlantHydraulicsMod.F90 and adds pFUnit verification tests.

Collaborators:

• @jpalex

Linked issues addressed, if any:

• Contributes to NGEET/fates#1409

Expectation of Answer Changes:

• Bit-for-Bit (B4B) with baseline master
• Roundoff-level differences only
• Expected Answer Changes (ECA)

Description of generative AI usage (as necessary)

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

Checklist

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

Test Results:

CTSM (or) E3SM (specify which) test hash-tag: N/A
CTSM (or) E3SM (specify which) baseline hash-tag: N/A
FATES baseline hash-tag: 19ee3688

Test Output:
[x] Executed pFUnit / CIME regression tests