Suppress kernel startup stdout during execution

[0xTaoZ]

Summary

Fixes #899.

Some kernels can write startup messages directly to the parent process' stdout while nbclient starts the kernel. When Papermill is writing the executed notebook JSON to stdout, that message can be prepended before the notebook content and make the output invalid JSON.

This keeps parent stdout quiet while the kernel context is active. Notebook stream output is still handled through Papermill's existing log_output, stdout_file, and notebook output paths.

Tests

• python -m pytest papermill/tests/test_clientwrap.py papermill/tests/test_cli.py -q
• python -m black --check --skip-string-normalization papermill/clientwrap.py papermill/tests/test_clientwrap.py
• python -m ruff check papermill/clientwrap.py papermill/tests/test_clientwrap.py
• python -m ruff format --check papermill/clientwrap.py papermill/tests/test_clientwrap.py
• pre-commit run --files papermill/clientwrap.py papermill/tests/test_clientwrap.py
• git diff --check HEAD~1..HEAD