Fix hf-xet version mismatch between install_requires and extras\n\nFixes #3239\n\nThe hf-xet package had inconsistent version requirements:\n- install_requires specified: hf-xet>=1.2.0,<2.0.0\n- extras['hf_xet'] specified: hf-xet>=1.1.3,<2.0.0\n\nThis caused version conflicts when installing huggingface-hub[hf_xet].\n\nFollowing maintainer feedback, implemented DRY principle by:\n- Creating HF_XET_VERSION variable with the version string\n- Using it in both install_requires and extras['hf_xet']\n\nThis ensures consistency and prevents future regressions. (#3662)