[XPU] Detect per-commit XPU tests dynamically in CI path filter

[arathi-hlab]

Motivation

The main_package path filter in pr-test-xpu.yml relies on a hand-kept list of test/registered/ files. New register_xpu_ci tests that aren't added to it (e.g. test_ngram_corpus.py from #31362) don't trigger XPU CI when changed.

Split out of #41075 (part 2/2). Part 1 (disable test_ngram_corpus + extend the explicit list) is #41224. The two PRs are independent.

Modifications

• check-changes now lists changed test/registered/**/*.py files (dorny/paths-filter with list-files: json).
• New scripts/ci/xpu/detect_xpu_test_changes.py parses them with ci_register.ut_parse_one_file (the parser run_suite.py uses) and outputs true if any has a non-nightly register_xpu_ci. Unparseable files also return true so run_suite surfaces the error. Stdlib-only; runs on ubuntu-latest.
• The result is OR-ed into main_package / changes_exist, so future XPU tests trigger XPU CI even if nobody updates the list.

Verification

Against the current tree, the script flags exactly the 42 files run_suite.py selects for stage-a-test-1-gpu-xpu,stage-b-test-1-gpu-xpu (39 enabled + 3 disabled), and nothing else. CUDA/AMD-only and nightly-only XPU files return false.

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CI States

Latest PR Test (Base): ✅ Run #36145538875
Latest PR Test (Extra): ❌ Run #36145538144
Latest PR Test (AMD ROCm 10): ❌ Run #36145538730

[arathi-hlab]

Closing as duplicate of #41075, which has the identical diff and existing review history. Please continue review there.