Add diffusivity implementation to VanGenuchten model

[gerlero]

No description provided.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.99%. Comparing base (b4ca0a3) to head (8db5d65).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
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- Coverage   86.25%   85.99%   -0.27%     
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  Files          13       13              
  Lines         553      557       +4     
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+ Hits          477      479       +2     
- Misses         76       78       +2     

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[Copilot]

🟢 Approval recommended

The analytic VanGenuchten.__call__ implementation is validated by tests against the prior K(θ)/C(θ) behavior including first and second derivatives across the domain.

Pull request overview

This PR adds an explicit (closed-form) moisture diffusivity implementation for the VanGenuchten Richards-based model, making VanGenuchten.__call__ compute diffusivity directly rather than via the generic K(θ)/C(θ) path.

Changes:

• Implement VanGenuchten.__call__ with an analytic expression for diffusivity D(θ).
• Add a regression test that validates D(θ) and its first/second derivatives against the existing K(θ)/C(θ) formulation across a representative theta_range.

File summaries

File	Description
src/frontx/models.py	Adds an analytic VanGenuchten.__call__ diffusivity implementation.
tests/test_models.py	Adds tests comparing analytic diffusivity (and its derivatives) to K/C.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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