fix(admin): handle xgrammar parser load failures

[stamsam]

Summary

• prevent /admin/api/grammar/parsers from returning 500 when xgrammar imports but its native binding cannot be loaded
• log the discovery failure and return an empty parser list, matching the existing optional-dependency behavior when xgrammar is absent
• add a regression test for the native binding load failure reported in Cannot find library: libxgrammar_bindings.dylib, libxgrammar_bindings.so #1005

Fixes #1005

Testing

• .venv/bin/python -m pytest tests/test_grammar.py::TestListGrammarParsers
• python3 -m py_compile omlx/admin/routes.py tests/test_grammar.py
• git diff --check