Replace defensive isinstance check with assertion in call_historic

Convert the isinstance(res, list) conditional to an assertion since
res is guaranteed to always be a list in call_historic:

1. Historic hooks are called with firstresult=False, which always
   returns a list from _multicall
2. Historic hooks cannot have wrappers/hookwrappers (enforced by
   validation), so no teardowns can modify the result type

This change:
- Documents the invariant explicitly
- Achieves 100% branch coverage on _hooks.py (was 99%)
- Makes the code clearer by removing unreachable defensive code
- Will catch future bugs if the invariant is violated

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: RonnyPfannschmidt

src/pluggy/_hooks.py

@@ -532,9 +532,11 @@ def call_historic(
         res = self._hookexec(self.name, self._hookimpls.copy(), kwargs, False)
         if result_callback is None:
             return
-        if isinstance(res, list):
-            for x in res:
-                result_callback(x)
+        # Historic hooks are called with firstresult=False and cannot have wrappers,
+        # so the result is always a list.
+        assert isinstance(res, list), "historic call should always return a list"
+        for x in res:
+            result_callback(x)
 
     def call_extra(
         self, methods: Sequence[Callable[..., object]], kwargs: Mapping[str, object]

testing/test_pluginmanager.py

@@ -219,12 +219,12 @@ def test_unregister_unknown_plugin_raises(pm: PluginManager) -> None:
     class Plugin1:
         @hookimpl
         def he_method1(self, arg):
-            return arg + 1
+            return arg + 1  # pragma: no cover
 
     class Plugin2:
         @hookimpl
         def he_method1(self, arg):
-            return arg + 2
+            return arg + 2  # pragma: no cover
 
     # Register Plugin1
     p1 = Plugin1()