Handle allocation failure when boxing

[maleadt]

Runtime.box wrote the type tag and payload through every pointer returned by malloc. OpenCL has no device allocator and returns C_NULL; after inlining, back-end optimizers can treat those stores as undefined and remove the throwing path, including signal_exception.

Use the same failure path as gc_pool_alloc: report OOM and throw OutOfMemoryError before writing. This prevents an invalid Julia object from escaping and gives optimization a real exceptional path. Cover both allocator-less SPIR-V back-ends and verify the OOM and original exception paths.

[maleadt]

This is a prerequisite for improved exception handling in OpenCL.jl (without having to add a device-side allocator).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.14%. Comparing base (c098b16) to head (06cc32c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #919      +/-   ##
==========================================
+ Coverage   85.04%   85.14%   +0.09%     
==========================================
  Files          29       29              
  Lines        5563     5566       +3     
==========================================
+ Hits         4731     4739       +8     
+ Misses        832      827       -5     

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