Fix Clang compiler warnings in fflib and PETSc/SLEPc plugins

[Copilot]

Several Clang warnings were being emitted across fflib and the MPI plugins. This PR addresses them with minimal, targeted fixes.

Changes

• src/fflib/Operator.hpp — Fix -Wnonnull: nullptr was being used where a value of type R is required in Op2_padd::f. Replaced with R() (zero-initialized default):

  // Before
  new R ((a ? *a : nullptr) + (b ? *b : nullptr))
  // After
  new R ((a ? *a : R()) + (b ? *b : R()))

• src/fflib/AFunction.hpp — Fix implicit const-pointer cast in memcpy: add explicit (void*) cast to the destination argument.

• plugin/mpi/PETSc-code.hpp / plugin/mpi/SLEPc-code.hpp — Suppress -Wunused-value warnings emitted from within the PETSc/SLEPc headers using #pragma clang diagnostic push/pop. Also normalizes the slepc.h include to use angle brackets (<slepc.h>).

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