docs(parser): Add doctest to _extract_mutex_groups

Add working doctests demonstrating:
- Extracting mutex groups from ArgumentParser
- Verifying actions map to same MutuallyExclusiveGroup instance
- Empty mapping for parsers without mutex groups

Uses identity checks (is) instead of set() since dataclasses
aren't hashable by default.

Author: tony

docs/_ext/sphinx_argparse_neo/parser.py

@@ -366,6 +366,35 @@ def _extract_mutex_groups(
     -------
     dict[int, MutuallyExclusiveGroup]
         Mapping from action id to the MutuallyExclusiveGroup it belongs to.
+
+    Examples
+    --------
+    Extract mutually exclusive groups from a parser with one group:
+
+    >>> parser = argparse.ArgumentParser()
+    >>> group = parser.add_mutually_exclusive_group()
+    >>> _ = group.add_argument("--foo", help="Use foo")
+    >>> _ = group.add_argument("--bar", help="Use bar")
+    >>> mutex_map = _extract_mutex_groups(parser)
+    >>> len(mutex_map)
+    2
+
+    Each action in the group maps to the same MutuallyExclusiveGroup:
+
+    >>> values = list(mutex_map.values())
+    >>> values[0] is values[1]
+    True
+    >>> len(values[0].arguments)
+    2
+    >>> [arg.names[0] for arg in values[0].arguments]
+    ['--foo', '--bar']
+
+    A parser without mutex groups returns an empty mapping:
+
+    >>> parser2 = argparse.ArgumentParser()
+    >>> _ = parser2.add_argument("--verbose")
+    >>> _extract_mutex_groups(parser2)
+    {}
     """
     mutex_map: dict[int, MutuallyExclusiveGroup] = {}