[RF] Refactor RooAICRegistry and its usage

[guitargeek]

Modernize RooAICRegistry, the helper that operator p.d.f.s use to cache analytical integration codes and their associated RooArgSets, and clean up its usage across RooFitCore.

Storage and ownership:

• Replace the four parallel std::vector<RooArgSet*> members and the hand-written copy/destroy bookkeeping with a single std::array<std::vector<std::unique_ptr<RooArgSet>>, 4>. The manual destructor and copyImpl() helper are gone; lifetime is now handled by the smart pointers.
• store() no longer takes ownership of the RooArgSets passed to it; it only snapshots them. Callers are updated accordingly: they either keep their unique_ptrs (RooAbsAnaConvPdf) or pass stack objects instead of heap allocations (RooAbsCachedPdf), removing several new/release()/delete dances.

API:

• Replace the four retrieve() out-parameter overloads with a single method returning a small Output struct (the code list plus the four set pointers), so call sites no longer declare and thread raw pointers by reference.
• Drop the ClassDef/dictionary and the corresponding LinkDef entry: all data members were transient, so the registry was never persisted and does not need I/O support.

Cleanups in the consumers:

• Remove the unused _codeReg registry members (and the now-redundant RooAICRegistry.h includes) from RooHistFunc, RooHistPdf and RooAddModel.
• In RooProdPdf, _genCode only ever stored code lists without any RooArgSets, so demote it from a RooAICRegistry to a plain std::vector<std::vector<int>> and look codes up directly.

AI was only used to generate the commit message, and all the code changes were done by the human.

[github-actions]

Test Results

    23 files      23 suites   3d 13h 44m 53s ⏱️
 3 852 tests  3 852 ✅ 0 💤 0 ❌
78 483 runs  78 483 ✅ 0 💤 0 ❌

Results for commit 06b35b1.