fix(fates): fix c13disc_acc weighting order and add EDAccumulateFluxes unit tests

[johnpaulalex]

Depends on parent PR: #1601

Description:

Fixes a calculation ordering bug in EDAccumulateFluxesMod.F90 where gpp_acc was accumulated prior to computing the c13disc_acc weighted mean, resulting in double-counting gpp_tstep in the denominator. Also updates the zero-denominator check to <= 0.0_r8 for negative total GPP underflow protection, adds a prt_global association safeguard in PRTGenericMod.F90, introduces shared test factory helpers (CreateTestSite and SetTestFluxes) in FatesFactoryMod.F90, and adds a full pFUnit test suite (test_EDAccumulateFluxes.pf) verifying multi-site iteration, multi-cohort/multi-layer accumulation, empty cohort linked lists, and underflow protections.

Specific notes

Collaborators:

• @jpalex

Linked issues addressed, if any:

• None

Expectation of Answer Changes:

• Bit-for-Bit (B4B) with baseline master
• Roundoff-level differences only
• Expected Answer Changes (ECA)
  • If ECA, describe exact conditions under which answers change and attach scientific evaluation summary/plots:
    Modifies carbon isotope discrimination accumulation (c13disc_acc) across timesteps for active vegetated canopy cohorts.

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: 5efa6fbfd173afe85d523431e75c8a6ec493b242
FATES baseline hash-tag: 960a4f1c

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