Add missing return type hints to BaseTokenizer.save, save_model, to_str

[RudrenduPaul]

What this PR does

Adds missing return type hints to three methods in
bindings/python/py_src/tokenizers/implementations/base_tokenizer.py:

• save_model(self, directory: str, prefix: Optional[str] = None) -> List[str]
• save(self, path: str, pretty: bool = True) -> None
• to_str(self, pretty: bool = False) -> str

This is a self-identified typing-consistency fix (no linked issue). Every
other method in this class already carries a return type hint; these three
were the only ones missing one. to_str's own docstring already documents
"Returns: str", so the annotation was simply never added to the signature.

Why these types are correct

Each method is a thin wrapper delegating to the compiled Rust bindings, and
the return type was verified against the auto-generated .pyi stubs
(bindings/python/py_src/tokenizers/__init__.pyi and
bindings/python/py_src/tokenizers/models.pyi), which are the source of
truth for the underlying Rust-exposed signatures:

• save_model returns self._tokenizer.model.save(directory, prefix=prefix).
  Model.save(self, /, folder: str, prefix: str | None = None, name: str | None = None) -> "list[str]"
  per models.pyi — hence List[str] (the file already imports List from
  typing).
• save returns self._tokenizer.save(path, pretty).
  Tokenizer.save(self, /, path: str, pretty: bool = True) -> "None" per
  __init__.pyi — hence None.
• to_str returns self._tokenizer.to_str(pretty).
  Tokenizer.to_str(self, /, pretty: bool = False) -> "str" per
  __init__.pyi — hence str, matching the existing docstring.

Scope note: not a duplicate of #1928 / issue #1927

#1928 (merged, closing issue #1927) added type hints
and an automatic stub generator for the compiled PyO3 bindings (.pyi
files, stub.py, stub-gen tool). It did not touch the pure-Python
implementations/ package, so base_tokenizer.py's three unannotated
methods were left untouched by that effort. Open PR #1932 ("fix: added type
hints in .py files") likewise only touches benches/examples/scripts/tests,
not base_tokenizer.py. Confirmed via gh pr view file lists before
starting this change.

No behaviour change

Type-hint-only change. No logic was modified.

Testing

• ruff check bindings/python/py_src/tokenizers/implementations/base_tokenizer.py — all checks passed.
• ruff format --check bindings/python/py_src/tokenizers/implementations/base_tokenizer.py — passes, file already formatted.
• python3 -c "import ast; ast.parse(...)" on the modified file — passes.

Note: Claude Code was used to assist in drafting this change. All
changes were reviewed by the submitter.