Fix non-conservative mortars for P4estMesh 3D

[amrueda]

A unique mortar flux used to be computed from the small interfaces and then projected to the big elements and copied to the small elements. This works for conservation laws. However, for non-conservative equations, the mortar flux should be computed from both sides of the mortar due to its asymmetric nature.

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.91%. Comparing base (4a81e00) to head (35aaf4e).

Files with missing lines	Patch %	Lines
src/solvers/dgsem_p4est/dg_3d_parabolic.jl	0.00%	2 Missing ⚠️

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Flag	BASE (4a81e00)	HEAD (35aaf4e)
unittests	26	25

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2127      +/-   ##
==========================================
- Coverage   96.36%   87.91%   -8.44%     
==========================================
  Files         477      477              
  Lines       37753    37757       +4     
==========================================
- Hits        36378    33194    -3184     
- Misses       1375     4563    +3188     

Flag	Coverage Δ
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[ranocha]

https://github.com/trixi-framework/Trixi.jl/actions/runs/11460837729/job/31888478768?pr=2127#step:7:11430 looks like a real error

[andrewwinters5000]

https://github.com/trixi-framework/Trixi.jl/actions/runs/11460837729/job/31888478768?pr=2127#step:7:11430 looks like a real error

It seems like the parabolic solver is attempting to use the surface_flux from the hyperbolic solver instead of BR1. This would (possibly) explain why it is throwing an error that flux does not exist for the equations_parabolic. Although I am not certain.

[andrewwinters5000]

I think I tracked down the issue. On TreeMesh there is a specialized version of calc_mortar_flux! that dispatches on the AbstractParabolicEquations type (https://github.com/trixi-framework/Trixi.jl/blob/main/src/solvers/dgsem_tree/dg_3d_parabolic.jl#L722-L726). However, no such equivalent is implemented for the P4estMesh, so I believe the parabolic solver on P4estMesh is actually grabbing the hyperbolic variant (https://github.com/trixi-framework/Trixi.jl/blob/main/src/solvers/dgsem_p4est/dg_3d.jl#L517-L521). In turn, I think this causes the issue of attempting to call the "wrong" flux later.