Add automatic generated fortran support for setting and returning contexts

Major cleanup of the code for setting and returning application (previously called user) contexts
- PetscCtx ctx is now t

Add automatic generated fortran support for setting and returning contexts

Major cleanup of the code for setting and returning application (previously called user) contexts
- PetscCtx ctx is now the standard for passing in contexts
- PetscCtxRt ctx is the standard for passing out contexts, including context destroy routines based on PetscCtxDestroyFn

Both of these are typedef to void*

Also removed most use of user and userctx for context arguments now consistently named ctx

Improved documentation on PetscCtx and PetscCtxRt and on how to use contexts (including getting them) from Fortran

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Merge remote-tracking branch 'origin/release'

MatCreateSubMatrix_Nest: need to increase the state of the object

- Replace MatMissingDiagonal() and MatMarkDiagonals_SeqXXX() with MatGetDiagonalMarkers_SeqXXX()

- Mat_SeqXXX->diag is not automatically created during MatAssemblyEnd() saving memory and time

- Replace MatMissingDiagonal() and MatMarkDiagonals_SeqXXX() with MatGetDiagonalMarkers_SeqXXX()

- Mat_SeqXXX->diag is not automatically created during MatAssemblyEnd() saving memory and time

- Accessing Mat_SeqXXX->diag now requires the use of MatGetDiagonalMarkers_SeqXXX() except when the current values are known to be correct; for example during numerical factorizations and solves

- Mat_SeqXXX->diag is now never shared among matrices; hence the free_diag flag is gone. That was always a risky proposition since any of the owning matrices could chang the values thus making them incorrect for other owners.

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Merge branch 'barry/2025-10-24/fix-dmshellsetdestroyctx' into 'main'

Finish converting the function prototypes of destroy for contexts to use PetscCtxDestroyFn

See merge request petsc/petsc!8810

Fix outdated output of MatView() for nest matrix with ASCII

Now matches VecNest version and does not have outdated reprinting of Mat Object and type

Change names of Mat_XXX product contexts to MatProductCtx_XXX for code maintainability

Update destroy callback of all MatProductCtx and MatShellSetMatProductOperation() to use PetscCtxDestroyFn

checkbadSource: enforce proper style in makefiles