ci(#2484): use 'pragma: lax no cover' for timing-dependent exception catches

The two catches I touched in the previous commit
(``except asyncio.CancelledError`` and
``except anyio.ClosedResourceError`` in the teardown result-collection
loop) turn out to be timing-dependent:

  - On Windows the teardown of the background reader/writer finishes
    before the loop reaches ``await task``, so ``await task`` never
    raises CancelledError / ClosedResourceError and the branches are
    unreached — needs pragma.
  - On some Linux runs the reader/writer is still running when we
    cancel and await, so the branches *are* reached — strict-no-cover
    complained that a plain ``pragma: no cover`` was wrong there.

Switched both to ``pragma: lax no cover``, which the project's
coverage config already defines as the opt-out marker for exactly
this case (exists on other lines in the same file, e.g. 247).
It is excluded from coverage reporting but does not trip
strict-no-cover when the line happens to be hit.

Author: sakenuGOD

src/mcp/client/stdio.py

@@ -158,9 +158,9 @@ def _on_done(task: asyncio.Task[None]) -> None:
         for task in tasks:
             try:
                 await task
-            except asyncio.CancelledError:
+            except asyncio.CancelledError:  # pragma: lax no cover - timing-dependent on teardown races
                 pass
-            except anyio.ClosedResourceError:
+            except anyio.ClosedResourceError:  # pragma: lax no cover - timing-dependent on teardown races
                 pass
             except BaseException as exc:  # noqa: BLE001
                 if pending_exc is None:  # pragma: no branch