fix: add coverage pragmas to _wait_for_file_growth safety paths

The timeout raise and file-not-yet-exists branch are safety nets that
don't execute in normal test runs. Mark them with coverage pragmas to
satisfy the 100% coverage requirement.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: BabyChrist666

tests/client/test_stdio.py

@@ -237,14 +237,14 @@ async def _wait_for_file_growth(
     prev_size = -1
 
     while time.monotonic() < deadline:
-        if os.path.exists(file_path):
+        if os.path.exists(file_path):  # pragma: no branch
             size = os.path.getsize(file_path)
             if size > 0 and prev_size >= 0 and size > prev_size:
                 return  # File is growing
             prev_size = size
         await anyio.sleep(poll_interval)
 
-    raise AssertionError(f"{description} did not start writing within {timeout}s")
+    raise AssertionError(f"{description} did not start writing within {timeout}s")  # pragma: no cover
 
 
 class TestChildProcessCleanup: