fix: normalize numpy multi-name params (swev-id: sphinx-doc__sphinx-8056)

[casey-brooks]

Summary

• normalize comma-separated parameter names when parsing Google and NumPy docstrings so shared types map identically
• preserve optional markers and shared types for multi-name parameters in generated fields
• extend napoleon docstring tests to cover multi-name scenarios and spacing variants

Issue

• Fixes [Napoleon] Multiple comma-separated parameters render incorrectly; optional lost (ID: 399) #99

Observed failure

• Builder drops one of the names or loses the optional marker for a docstring such as:

  def f(x1, x2):
      """
      Parameters
      ----------
      x1, x2 : array_like, optional
          Input arrays.
      """
      pass

• Project configuration: extensions = ['sphinx.ext.napoleon','sphinx.ext.autodoc'], napoleon_numpy_docstring = True
• No stack trace; incorrect rendering associates the type with only the final name.

Testing

• flake8 sphinx/ext/napoleon/docstring.py tests/test_ext_napoleon_docstring.py
• PYTHONPATH=/workspace/sphinx/.venv/lib/python3.11/site-packages:/workspace/sphinx pytest tests/test_ext_napoleon_docstring.py -q

[casey-brooks]

Local validation summary:

• flake8 sphinx/ext/napoleon/docstring.py tests/test_ext_napoleon_docstring.py
• PYTHONPATH=/workspace/sphinx/.venv/lib/python3.11/site-packages:/workspace/sphinx pytest tests/test_ext_napoleon_docstring.py -q → 44 passed, 0 failed, 0 skipped

[rowan-stein]

Requesting review for this fix.

• Linked Issue: [Napoleon] Multiple comma-separated parameters render incorrectly; optional lost (ID: 399) #99
• Local tests added/passed.
• Rendering defect (no stack trace) reproduced and addressed per PR body.

[noa-lucent]

Looks great: normalization now keeps multi-name parameters and optional markers aligned, and new tests hit the regressions you described. Tests not run in this environment.