Exempt the new nested async with lines from branch coverage on 3.14

The 3.14 jobs failed the 100% gate while every other version passed. Both
files were at 100% statement coverage; the shortfall was two partial
branches, one per file, on the `async with stdio_client(...)` line nested
inside `anyio.fail_after(...)`.

On 3.14 the tracer records a self-arc for that line instead of the arcs
into the body and out past the enclosing block, so both declared exits
read as missing even though the body demonstrably runs. Same code and
same tests measure 100% on 3.13.

This is the existing convention here: test_lifecycle.py already carries
`# pragma: no branch` on an identical `stdio_client` line, and the repo
uses the pragma in 232 places, including one that names the same cause.

Author: maisymylod

tests/client/test_stdio.py

@@ -1033,7 +1033,8 @@ async def test_a_closed_errlog_stops_stderr_forwarding_without_failing_the_sessi
 
     with caplog.at_level(logging.DEBUG, logger="mcp.client.stdio"):
         with anyio.fail_after(5):
-            async with stdio_client(FAKE_PARAMS, errlog=cast(TextIO, errlog)) as (read_stream, _):
+            # no branch: coverage mis-traces the exit arcs of a nested `async with` on 3.14.
+            async with stdio_client(FAKE_PARAMS, errlog=cast(TextIO, errlog)) as (read_stream, _):  # pragma: no branch
                 await process.feed_stderr(b"server diagnostics no one will read\n")
                 await errlog.attempted.wait()
 

tests/transports/stdio/test_lifecycle.py

@@ -238,7 +238,8 @@ async def test_server_stderr_output_reaches_an_errlog_without_a_file_descriptor(
         # The bound covers one interpreter cold start on a loaded runner; a
         # healthy run takes well under a second.
         with anyio.fail_after(10.0):
-            async with stdio_client(params, errlog=errlog):
+            # no branch: coverage mis-traces the exit arcs of a nested `async with` on 3.14.
+            async with stdio_client(params, errlog=errlog):  # pragma: no branch
                 stream = await accept_alive(sock)
                 stack.push_async_callback(stream.aclose)