IniSource.get_loader: check section.prefix

ensure that loaders returned from .ini source are bound
to the correct section prefix, if specified.

add comment explaining why the code must look up the name
in the _section_mapping

fix tox-dev#2926

src/tox/config/source/ini.py

@@ -40,8 +40,13 @@ def sections(self) -> Iterator[IniSection]:
             yield IniSection.from_key(section)
 
     def get_loader(self, section: Section, override_map: OverrideMap) -> IniLoader | None:
-        sections = self._section_mapping.get(section.name)
-        key = sections[0] if sections else section.key
+        # look up requested section name in the generative testenv mapping to find the real config source
+        for key in self._section_mapping.get(section.name) or []:
+            if section.prefix is None or Section.from_key(key).prefix == section.prefix:
+                break
+        else:
+            # if no matching section/prefix is found, use the requested section key as-is (for custom prefixes)
+            key = section.key
         if self._parser.has_section(key):
             return IniLoader(
                 section=section,