ci(#2484): drop pragmas on lines strict-no-cover says are reached

Third CI round: coverage reached 100.00% on the previous commit, but
the project also runs ``strict-no-cover`` which complains when a
line is marked ``# pragma: no cover`` yet actually gets hit by the
test suite (it prevents coverage rot from creeping in under pragma
cover).

It flagged two of my pragmas:
  src/mcp/client/stdio.py:161 — `except asyncio.CancelledError:`
  src/mcp/client/stdio.py:163 — `except anyio.ClosedResourceError:`

Both are reached on Linux during the normal teardown of the new
stdio tests, so the pragma was wrong. Removed them. (The Windows
runs did not hit these branches — which explains why Windows matrix
passed last round but Linux did not; strict-no-cover runs on the
Linux jobs.)

Kept the pragmas on lines 131, 155, 166 — those are the defensive
branches that still are not reached by any existing test on either
platform and still warrant the marker.

15 stdio tests pass locally; ruff + pyright clean.

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:  # pragma: no cover - only if a task was still pending above
+            except asyncio.CancelledError:
                 pass
-            except anyio.ClosedResourceError:  # pragma: no cover - swallowed by reader/writer already
+            except anyio.ClosedResourceError:
                 pass
             except BaseException as exc:  # noqa: BLE001
                 if pending_exc is None:  # pragma: no branch