Add verification to the specialized material pipeline cache

## What problem does this solve or what need does it fill?

The renderer keeps a cache of specialized material pipelines as an optimization - see `SpecializedMaterialPipelineCache` and variants. But it's easy for the cache to accidentally become stale and cause issues that are hard to debug, like #18712 and #23118.

## What solution would you like?

There should be a feature flag and/or runtime toggle that enables cache verification. So when specialization gets a cache hit, it should continue to recalculate the pipeline and verify it's the same as the cached pipeline. Ideally it would also log details on what inputs were different, like `MeshPipelineKey`.

Currently the cache logic is semi-duplicated in several specialization functions - `specialize_material_meshes`, `specialize_prepass_material_meshes`, and `specialize_shadows`. So some refactoring might be needed if they want to share verification.

## What alternative(s) have you considered?

Add a toggle that simply disables the cache. This is more limited, but still useful for testing.