Spool native warnings without pipes during EP compilation

[KayMKM]

Summary

• replace the pipe used by native warning suppression with a file-backed temporary spool
• filter and replay preserved native diagnostics only after restoring stderr
• retain fail-open behavior, Windows handle restoration, warning filtering, and bounded memory usage

Root cause

The warning filter introduced in #1246 redirected native stderr to a pipe. VitisAI can hang inside ort.InferenceSession when its compiler sees that pipe handle. The reader was draining correctly, so this was not the full-buffer deadlock fixed by #1223; changing the reader to defer replay still hung, which isolated the pipe handle itself as the trigger.

A temporary file preserves warning filtering without pipe semantics or a finite producer buffer. With an empty VAIP cache and no VitisAI-specific bypass, facebook/convnext-tiny-224 completed on VitisAI NPU in 144.1 seconds. Disabling warning filtering entirely completed the same workload in 145.1 seconds.

Validation

• uv run --no-sync pytest tests/unit/utils/test_native_stderr.py tests/unit/commands/test_perf_cli.py -q --basetemp temp/pytest_tmp/native-warning-file-backed-final (143 passed, 1 platform skip)
• uvx ruff check src/winml/modelkit/utils/native_stderr.py tests/unit/utils/test_native_stderr.py src/winml/modelkit/commands/perf.py tests/unit/commands/test_perf_cli.py
• cold-cache VitisAI NPU perf with warning filtering enabled: PASS in 144.1s