test(fates): Add EDAccumulateFluxesMod unit tests and document magic constants

[johnpaulalex]

Description of changes

This PR adds comprehensive pFUnit unit tests for EDAccumulateFluxesMod covering carbon, respiration, N-fixation, C13 discrimination, and net uptake accumulation logic across normal, zero-GPP, subnormal floating point, multi-cohort, multi-patch, multi-site, multi-PFT, and zero-layer edge cases.

All magic constants, filter flags (3/1), sentinels (999.0_r8), and unit fixture rates in the test file include one-line explanatory comments per project documentation rules. Additionally, library path fallbacks were added to the unit test builder framework.

Specific notes

Contributors other than yourself, if any:

• @johnpaulalex

Linked issues addressed, if any:

• None

Description of generative AI usage:

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

Answer Changes & Scientific Impact

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

User Interface & Namelist Changes

• Namelist / Defaults modified? No
• XML / Build script changes? No

Testing planned or performed, if any:

• Executed pFUnit / CIME regression tests

CTSM / CESM baseline hash-tag: 33155f41
PR branch hash-tag: ad0d728e

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 / unit test addition; no documentation updates required.

[johnpaulalex]

Closed in favor of updated PR #1598.