Gate operator correctness tests behind ARK_UT_CORRECTNESS=1 env var

[chhwang]

Gate operator correctness tests behind ARK_UT_CORRECTNESS=1 env var

op_test() calls — GPU execution with numerical comparison against CPU
baselines — now check ARK_UT_CORRECTNESS=1 at runtime. When unset,
op_test() prints [SKIP] and returns immediately. Model-validation
tests (graph construction, shape inference, error paths) always run.

Changes:

• ark/ops/ops_test_common.cpp: early-return in op_test() when
  ARK_UT_CORRECTNESS is not "1".
• ark/CMakeLists.txt: CTest label correctness on the 11 test
  binaries that call op_test().
• .github/workflows/ut.yml: two-step ctest — step 1 runs all 37
  binaries (correctness tests hit the skip path, ~0.03 s each); step 2
  runs ARK_UT_CORRECTNESS=1 ctest -L correctness for full GPU
  validation. gcov merges .gcda counters across both runs, so
  coverage captures both code paths.

No tests deleted or weakened. CI runs both steps on every push and PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.89%. Comparing base (0559ce2) to head (a586ac2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #251      +/-   ##
==========================================
+ Coverage   86.85%   86.89%   +0.04%     
==========================================
  Files         125      125              
  Lines        6099     6113      +14     
==========================================
+ Hits         5297     5312      +15     
+ Misses        802      801       -1     

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