In this update I have added tests for the silent flag in execute_request, particularly within the context of subshells and concurrent execution.

The silent flag is a critical feature for controlling output in clients like Jupyter notebooks. While the core functionality was previously addressed, there were no specific tests to ensure its reliability in more complex scenarios involving subshells.

This commit adds a new test case, test_silent_flag_in_subshells, to tests/test_subshells.py which:

• Verifies that silent=True correctly suppresses all output-related IOPub messages (like stream or execute_result) for code executed in the main shell.
• Confirms the same behavior for code executed in a subshell.
• Includes a test for concurrent execution, ensuring a silent request in one subshell does not interfere with a non-silent, output-producing request in the main shell.
• Refines the test logic to correctly handle expected status messages, which are sent regardless of the silent flag, while ensuring no other output is generated.

Ensuring the silent flag's reliability in subshells is crucial for preventing unexpected output flooding in notebooks and other clients. These tests lock in the correct behavior, improve the project's test coverage, and will prevent future regressions in this critical area, leading to a more predictable and improved user experience.