`testpaths` is incorrectly placed in `[project.optional-dependencies]` section in python/pyproject.toml

**Describe the bug**

In `python/pyproject.toml`, the `testpaths` configuration is incorrectly placed under the `[project.optional-dependencies]` section. This is a pytest configuration option that should be under `[tool.pytest.ini_options]` section according to [pytest documentation](https://docs.pytest.org/en/stable/reference/customize.html#pyproject-toml).

* pytest may not recognize the testpaths configuration
- Tools that parse pyproject.toml may fail to parse because of an unknown key in `[project.optional-dependencies]`. Actually, `uv` generates a weired message because of this problem.

**To Reproduce**

- Tested on Ubuntu 24.04, Python 3.14.0, uv  0.9.4
- Actual command line or python code to reproduce the bug: `uv run python --version`

Following error will be shown:

```
  × No solution found when resolving dependencies for split (markers: python_full_version >= '3.10'):
  ╰─▶ Because there are no versions of test and sentencepiece[testpaths] depends on test, we can conclude that sentencepiece[testpaths]'s requirements are unsatisfiable.
      And because your project requires sentencepiece[testpaths], we can conclude that your project's requirements are unsatisfiable.
```

**Expected behavior**

Just a python version string, e.g.

```
Python 3.14.0
```

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