Replace complex _multicall wrapper logic with streamlined completion hook pattern

The _multicall function had intricate generator-based wrapper teardown logic
that was difficult to understand and maintain. This refactoring introduces
a completion hook pattern that simplifies the wrapper invocation process.

Key changes:
- Add CompletionHook type alias with (result, exception) -> (result, exception) signature
- Implement setup_and_get_completion_hook() method in WrapperImpl class
- Unify old-style and new-style wrapper handling using run_old_style_hookwrapper adapter
- Replace complex teardown loop with simple completion hook calls
- Adjust warning stacklevel from 6 to 7 for correct source attribution

The completion hook pattern makes wrapper teardown explicit and easier to debug
while maintaining full backward compatibility with both hookwrapper and wrapper
implementations.

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

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

Author: RonnyPfannschmidt

src/pluggy/_hook_callers.py

@@ -62,6 +62,12 @@ def _insert_hookimpl_into_list(
 
 _T_HookImpl = TypeVar("_T_HookImpl", bound="HookImpl")
 
+# Type alias for completion hook functions
+CompletionHook = Callable[
+    [object | list[object] | None, BaseException | None],
+    tuple[object | list[object] | None, BaseException | None],
+]
+
 
 def _insert_hookimpl_into_list(
     hookimpl: _T_HookImpl, target_list: MutableSequence[_T_HookImpl]