Fix conditional wrappers for vararg runtime calls

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

Summary

• Builds conditional execution wrappers from call-site argument types when the callee has fewer fixed LLVM parameters than the runtime activity comparison index, as happens for vararg Julia runtime calls like dynamic setproperty!.
• Adds an isolated type-unstable setproperty! regression test covering the BoundsError introduced after Fix loosen requirements for absint of newstruct #3286.

Local verification

timeout 3600 /home/crackauc/.juliaup/bin/julia +1.11 --project=test test/runtests.jl typeunstable

Test Summary: | Pass  Total   Time
  Overall     |   26     26  38.4s
    SUCCESS

Notes

The original SciML issue workload gets past the previous BoundsError with this patch, but still later fails with a separate IllegalTypeAnalysisException in SciMLSensitivity.ReverseLossCallback.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.29%. Comparing base (e619385) to head (3aa1eb9).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gradientutils.jl	90.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3290   +/-   ##
=======================================
  Coverage   74.28%   74.29%           
=======================================
  Files          66       66           
  Lines       22602    22609    +7     
=======================================
+ Hits        16791    16797    +6     
- Misses       5811     5812    +1     

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