ci: make shadow workflow plugin-aware

[voltjia]

Summary

Wire the plugin test matrix helper into the shadow CI workflow without changing the main CI workflow.

This PR is stacked on #640 (build/external-plugin-roots).

Changes

• Add a plugin-matrix job to .github/workflows/ci_v2_shadow.yml.
• For pull requests, compute changed paths and select active hardware platforms through scripts/infini_ops_plugin_test_matrix.py --github-output.
• Keep manual dispatch behavior, including explicit platform selection and all.
• Skip the reusable shadow CI job when the selected platform list is empty.

Validation

• .venv/bin/python -m pytest -s -q tests/test_plugin_registry.py tests/test_plugin_test_matrix.py tests/test_generate_wrappers_plugins.py
• .venv/bin/ruff check scripts/infini_ops_plugin_registry.py scripts/infini_ops_plugin_test_matrix.py scripts/generate_wrappers.py tests/test_plugin_registry.py tests/test_plugin_test_matrix.py tests/test_generate_wrappers_plugins.py
• .venv/bin/python -m py_compile scripts/infini_ops_plugin_registry.py scripts/infini_ops_plugin_test_matrix.py scripts/generate_wrappers.py
• .venv/bin/python -c "import yaml; yaml.safe_load(open('.github/workflows/ci_v2_shadow.yml', encoding='utf-8'))"
• git diff --check

Full-platform validation

Full stack validation was run on the top branch build/external-plugin-source-paths at pre-style-rebase commit 907eff70; see #643 validation comment: #643 (comment). The later rebase only updates PR metadata and diagnostic wording for CONTRIBUTING.md compliance.

• NVIDIA: build passed; pytest failed with 1 failed, 3687 passed, 4404 skipped due to CUDA OOM in tests/test_torch_ops.py::test_op[..., svd] on the PyTorch reference path.
• MetaX: 3183 passed, 3400 skipped.
• Iluvatar: 2689 passed, 3894 skipped.
• Moore: passed after preloading the container OpenMP runtime, 2949 passed, 3643 skipped.
• Cambricon: 1781 passed, 4694 skipped.
• Ascend: pytest passed with 3359 passed, 3233 skipped, but the container exited with code 137; treat as not fully green by the quality gate.