Fix invalid setfield! fallback in MOIOptimizationNLPCache setproperty!

`MOIOptimizationNLPCache` is an immutable struct whose `setproperty!` delegates
mutation to the (mutable) evaluator and its `ReInitCache`. The final fallback
called `setfield!(cache, name, x)` on the immutable cache itself, which can never
succeed and which JET flagged as a guaranteed error (`setfield!: immutable struct
... cannot be changed`). Replace the dead fallback with an explicit, informative
`ArgumentError`.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RtJAi7bALH5XkakjS6Yb6j

Author: 3 people

lib/OptimizationMOI/src/nlp.jl

@@ -48,7 +48,9 @@ function Base.setproperty!(cache::MOIOptimizationNLPCache{E}, name::Symbol, x) w
     elseif name in fieldnames(OptimizationBase.ReInitCache)
         return setfield!(cache.evaluator.reinit_cache, name, x)
     end
-    return setfield!(cache, name, x)
+    # The cache itself is immutable; its own fields delegate reads through
+    # `getproperty` but are never reassigned, so there is no valid setter.
+    throw(ArgumentError(lazy"field `$name` of `MOIOptimizationNLPCache` is immutable and cannot be set"))
 end
 
 function SciMLBase.get_p(