test(ep-cpu): report the affinity-capability skip instead of returning silently

[justinchuby]

a_successful_affinity_call_is_never_reported_as_an_unsupported_target returns silently when allowed_cpus() is None. Eight lines so that it says so.

The test's only failure mode is a target that masks successfully while AFFINITY_MASKING_SUPPORTED says it cannot. A run that never reached the call therefore proves nothing — but a bare return produces a log identical to a check that ran and passed. That is the fail-open shape this test was itself written to prevent, sitting in the test.

Honest scope

• This is a log-only change. No assertion is added or altered.
• The new branch does not execute on Linux CI: allowed_cpus() returns Some here, so the skip is not taken. I verified the test still passes (1 passed, and no skip line printed, which is the expected result and also the proof the branch was not hit). I am not claiming this line is covered — I am claiming that if it is ever hit, it will be visible.

Commands

cargo fmt --all -- --check                                    # clean
cargo test --locked -p onnx-runtime-ep-cpu --lib \
  a_successful_affinity_call_is_never_reported_as_an_unsupported_target -- --nocapture
# test result: ok. 1 passed; 0 failed

Salvaged from #2084, which I am closing as superseded by #2034.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.18%. Comparing base (272bd94) to head (b0f59c8).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2119      +/-   ##
==========================================
+ Coverage   80.71%   81.18%   +0.46%     
==========================================
  Files         415      429      +14     
  Lines      205260   215175    +9915     
  Branches   205260   215175    +9915     
==========================================
+ Hits       165681   174686    +9005     
- Misses      34018    34717     +699     
- Partials     5561     5772     +211     

Flag	Coverage Δ
cli-ort-linux	72.51% <ø> (?)
cli-ort-windows	72.01% <ø> (?)
mlas	85.90% <ø> (?)
offline	81.32% <100.00%> (+0.60%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crates/onnx-runtime-ep-cpu/src/decode_affinity.rs	94.51% <100.00%> (-0.20%)	⬇️

... and 40 files with indirect coverage changes

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