fix: add Python 3.10 compat for BaseExceptionGroup import

On Python < 3.11, BaseExceptionGroup and ExceptionGroup are not builtins.
Import them from the exceptiongroup backport package conditionally.
Also fix coverage miss on unreachable pytest.fail line and revert
unnecessary builtins ruff config.

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

Author: Varun Sharma

pyproject.toml

@@ -126,7 +126,6 @@ executionEnvironments = [
 line-length = 120
 target-version = "py310"
 extend-exclude = ["README.md", "README.v2.md"]
-builtins = ["BaseExceptionGroup", "ExceptionGroup"]
 
 [tool.ruff.lint]
 select = [

src/mcp/shared/_task_group.py

@@ -12,11 +12,15 @@
 
 from __future__ import annotations
 
+import sys
 from types import TracebackType
 
 import anyio
 from anyio.abc import TaskGroup
 
+if sys.version_info < (3, 11):
+    from exceptiongroup import BaseExceptionGroup
+
 
 def collapse_exception_group(exc: BaseExceptionGroup) -> BaseException:  # type: ignore[type-arg]
     """Unwrap nested single-exception BaseExceptionGroups.

tests/shared/test_task_group.py

@@ -1,14 +1,20 @@
 """Tests for mcp.shared._task_group — collapsing ExceptionGroup wrapper."""
 
+import sys
+
 import anyio
 import pytest
+from anyio.abc import TaskStatus
 
 from mcp.shared._task_group import (
     _CollapsingTaskGroup,
     collapse_exception_group,
     create_mcp_task_group,
 )
 
+if sys.version_info < (3, 11):
+    from exceptiongroup import BaseExceptionGroup, ExceptionGroup
+
 # ---------------------------------------------------------------------------
 # collapse_exception_group unit tests
 # ---------------------------------------------------------------------------
@@ -121,7 +127,7 @@ async def test_cancel_scope_is_delegated() -> None:
 async def test_start_delegates_to_task_group() -> None:
     """start() delegates to the underlying task group."""
 
-    async def task_with_status(*, task_status: anyio.abc.TaskStatus[str] = anyio.TASK_STATUS_IGNORED) -> None:
+    async def task_with_status(*, task_status: TaskStatus[str] = anyio.TASK_STATUS_IGNORED) -> None:
         task_status.started("ready")
         await anyio.sleep(999)
 
@@ -144,15 +150,12 @@ async def test_task_group_not_entered_raises() -> None:
 @pytest.mark.anyio
 async def test_collapsed_exception_preserves_cause_chain() -> None:
     """The collapsed exception has the original ExceptionGroup as __cause__."""
-    try:
+    with pytest.raises(RuntimeError, match="root cause") as exc_info:
         async with create_mcp_task_group() as tg:
 
             async def failing() -> None:
                 raise RuntimeError("root cause")
 
             tg.start_soon(failing)
-    except RuntimeError as exc:
-        assert isinstance(exc.__cause__, BaseExceptionGroup)
-        assert str(exc) == "root cause"
-    else:
-        pytest.fail("Expected RuntimeError")
+
+    assert isinstance(exc_info.value.__cause__, BaseExceptionGroup)