Handle needs_functions callables without __name__ and non-iterable configurations (#2073)

[wayu15244]

Summary

Closes #2073.

Problem / Root Cause

In sphinx_needs/needs.py:merge_default_configs, registering functions from needs_config._functions had two failure modes:

1. _Config.add_function(self, function, name=None) directly accessed function.__name__. When given callable objects that do not have __name__ directly attached (such as functools.partial or callable class instances), it raised AttributeError: 'functools.partial' object has no attribute '__name__'.
2. If needs_functions was configured as None or a bare callable instead of a list/tuple, iterating over needs_config._functions raised TypeError: 'NoneType' object is not iterable or 'function' object is not iterable.

Solution

• In sphinx_needs/config.py: use getattr(function, '__name__', None) in add_function. If name is omitted and the callable has no __name__, emit a needs.config warning and return instead of raising an unhandled AttributeError.
• In sphinx_needs/needs.py: check isinstance(needs_config._functions, (list, tuple)) before iterating. For each entry, check getattr(needs_func, '__name__', None) and emit an informative warning advising users to provide a callable with __name__ or register via add_dynamic_function(func, name=...).
• Allows add_dynamic_function to successfully register callables without __name__ (such as functools.partial) when an explicit name is provided.

Tests Added

• test_needs_functions_entry_without_name_warns_and_is_ignored: verifies a callable without __name__ in needs_functions warns and is ignored without crashing.
• test_needs_functions_none_value_does_not_crash: verifies needs_functions = None does not crash.
• test_needs_functions_bare_callable_value_does_not_crash: verifies needs_functions = func does not crash.
• test_add_dynamic_function_allows_callables_without_name: verifies registering a functools.partial via add_dynamic_function(..., name='partial_func') executes and evaluates correctly in needs.

Validation

• uv run pytest packages/sphinx-needs/tests/test_needs_from_toml_functions.py: 14 passed.
• uv run pytest packages/sphinx-needs/tests/test_dynamic_functions.py: 37 passed.