fix(ep-cpu): make the dispatcher-placement probes inert under Miri (main is red, mine)

[justinchuby]

Main is red on Miri and it is mine

Reporting this against myself. #1915 merged at 01:59:54Z on required checks
(Fast (Linux x86_64), Rust quality) while Miri unsafe-crate soundness
was still running, and it subsequently failed. Miri is not a required
check on this repo, so auto-merge fired legitimately — no --admin, no
ruleset bypass — but the outcome is the same as if it had been bypassed: a
defect landed on main that CI caught. I had the fix pushed to the PR branch at
02:03, four minutes too late.

The defect. The Miri job runs
decode_spmd::tests::a_panic_in_the_dispatcher_shard_still_waits_for_the_workers.
That test dispatches on a real pool, and #1915 made dispatch call
sample_dispatcher_cpu() → libc::sched_getcpu(), which Miri has no shim for:

error: unsupported operation: can't call foreign function `sched_getcpu` on OS `linux`
    --> crates/onnx-runtime-ep-cpu/src/decode_spmd.rs:1607:32

A panic-safety test failing for reasons that have nothing to do with panic
safety.

The fix. sample_dispatcher_cpu, current_thread_os_id and the
sched_setaffinity call are gated on not(miri). Nothing is lost: a
CPU-placement sample is meaningless under an interpreter that does not model
CPUs, there is no /proc for a tid to index, and the property Miri exists to
check — that the unsafe blocks are sound — does not depend on the calls being
made. Setting ONNX_GENAI_CPU_DECODE_DISPATCHER_PIN under Miri now degrades to
"not pinned" instead of failing an unrelated test.

Verified with the workflow's exact invocation, on this branch, off current
main:

MIRIFLAGS="-Zmiri-disable-isolation -Zmiri-num-cpus=4 -Zmiri-ignore-leaks" \
  cargo +nightly miri test --locked -p onnx-runtime-ep-cpu --lib \
  decode_spmd::tests::a_panic_in_the_dispatcher
→ test result: ok. 1 passed; 0 failed

What I'm taking from it. "Wait for required CI" is not sufficient when a
job that can fail is not in the required set. For anything that adds an FFI
call inside a code path a Miri test executes, the check to run before merging
is Miri itself, locally, not the required set. I should have run it before
opening #1915 — the test is named in .github/workflows/miri.yml:227 and I
touched the exact function it exercises.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.32%. Comparing base (cdc7d93) to head (f299761).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/onnx-runtime-ep-cpu/src/decode_spmd.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1921      +/-   ##
==========================================
+ Coverage   80.14%   80.32%   +0.17%     
==========================================
  Files         413      415       +2     
  Lines      200822   204575    +3753     
  Branches   200822   204575    +3753     
==========================================
+ Hits       160957   164333    +3376     
- Misses      34343    34669     +326     
- Partials     5522     5573      +51     

Flag	Coverage Δ
cli-ort-linux	72.51% <ø> (?)
cli-ort-windows	72.01% <ø> (ø)
mlas	85.20% <ø> (?)
offline	80.45% <0.00%> (+0.09%)	⬆️

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_spmd.rs	89.82% <0.00%> (-0.75%)	⬇️

... and 26 files with indirect coverage changes

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