Add PhayaThaiBERT POS tagger (ONNX and Transformers) - #1550

The published tokenizer.json truncates at 510 tokens, so long input
was cut before chunking and every word past the cut was tagged X.
Turn off the tokenizer's built-in truncation and padding so chunking
by whole words takes effect.

- Tag words made only of whitespace or format characters (such as
  the zero-width space) as PUNCT without passing them to the model.
- Require onnxruntime>=1.12.0 (opset 17) and tokenizers>=0.19.0
  (the Metaspace format in tokenizer.json).
- Pin the default revision only for the default repository.
- Guard the lazily created tagger with a lock.
- Name huggingface-hub in the import check.
- List the phayathaibert_onnx extra in the installation notes.
- Test tags past the first 510 tokens, an over-long word, and format
  characters.

- Use D213 docstrings and typed fields in phayathaibert_onnx, and add
  an __init__ docstring (D107).
- Test PhayaThaiBERTTagger, tag() and the new pos_tag and
  pos_tag_transformers branches with a fake model, so CI covers the
  new code without onnxruntime, tokenizers or transformers.
- Fake every dependency inside patch.dict(sys.modules), so no real
  module (such as numpy) is imported and then unloaded by the test.
- Merge the _chunk_spans tests into one table-driven test.

The model accepts 512 positions, but its accuracy drops sharply from
position 415 on, in the original PyTorch model as well as the ONNX
export. With chunks of up to 510 tokens, tagging the whole UD Thai-TUD
test set as one input gave 80.26% accuracy; with 415 it gives 90.08%
(91.21% sentence by sentence).

The long-input test now checks that every repeat of a sentence gets
the same tags as the sentence alone, which fails with the old limit.

Only README.md changed on the Hugging Face repository; model.onnx, tokenizer.json and config.json are the same files.

- Check dependencies with importlib.util.find_spec instead of unused
  imports (pylint W0611).
- Use a one-line module docstring (pydocstyle D212); the model
  description moves to the class docstring.
- Mark the unused corpus parameter of tag() for SonarCloud (S1172).
- Tests: end a docstring with a period (D415), keep one assertion
  argument order (S3415), and use assertLessEqual (S5906).

CI runs mypy on tests too (mypy 1.19.1, Python 3.9), which rejects calls to the untyped fake model helpers.

Fixes pylint W0404 (reimport) reported by Codacy.

- Write zero-width and special space characters in tests as \u escapes,
  so they are visible and editors cannot strip them (pylint E2515).
- Rename a loop variable that shadowed the module-level tag() (W0621).
- Correct the length comment: the model runs on up to 510 tokens.

- Build the truncated word_ids list by unpacking, not concatenation.
- Annotate the shared WORDS test attribute as ClassVar.