fix: rename _cancelled to _ to satisfy pyright reportUnusedVariable

The split() return value for the cancelled subgroup is intentionally
discarded.  Use bare _ instead of _cancelled so pyright strict mode
recognises it as an unused-by-design binding.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

src/mcp/shared/_exception_utils.py

@@ -46,7 +46,7 @@ def collapse_exception_group(
     # split(type) uses isinstance on leaf exceptions, NOT on the group.
     # Using split(lambda) is incorrect because the lambda would first be
     # called on the group object itself.
-    _cancelled, non_cancelled = eg.split(cancelled_type)
+    _, non_cancelled = eg.split(cancelled_type)
 
     if non_cancelled is None:
         # Every exception is a cancellation – surface just one.