fix: pin plugin dependencies broken by upstream releases

[KartikP]

Three plugins break against newly released third-party packages. All three failures showed up in the CI for #420. That PR edits model_helpers, which makes CI test every language plugin, and none of the failures involve its diff.

Failures and fixes

• wikitext data: the plugin requires datasets==2.*, which does not cap pyarrow.
  • pyarrow 26.0.0 (released 2026-10-09) raises ImportError: pyarrow requires NumPy 2.0 or newer against our numpy<2 pin.
  • This fails tests/test_model_helpers/test_huggingface.py::TestNextWord::test_0_dimensional_tensor (Non-Plugins) on any PR that runs it.
  • Fix: pin pyarrow<26.
• earley_parser: NLTK 3.10 refuses to read corpus directories that are not listed in nltk.data.path.
  • Loading the bundled treebank fails with PermissionError: Security Violation [CorpusReader.__init__]: Unauthorized path .../earley_parser/treebank.
  • Fix: add the treebank directory to nltk.data.path before reading it.
• lm1b: an unpinned tensorflow install upgrades NumPy to 2.x.
  • The plugin's xarray import then fails at test collection with AttributeError: np.unicode_ was removed in the NumPy 2.0 release.
  • Fix: add numpy<2.

Verification

• pyarrow==26.0.0 with numpy==1.26.4 reproduces the ImportError. pyarrow==25.0.1 with the same NumPy imports cleanly.
• nltk==3.10.3 reproduces the PermissionError on the bundled treebank. With the path added, all 11 trees in sample_treebank parse.
• pip install --dry-run for linux x86_64 / Python 3.11:
  • tensorflow alone resolves to numpy 2.4.6, which reproduces the lm1b failure.
  • tensorflow with numpy<2 resolves to tensorflow 2.20.0 and numpy 1.26.4.
  • datasets==2.* with pyarrow<26 and numpy<2 resolves to datasets 2.21.0, pyarrow 25.0.1 and numpy 1.26.4.
• The model tests themselves have not been run locally. This PR touches all three plugins, so CI runs their tests.