fix: add warning log when using internal default simulator config

[FedericoMusa]

Description

Following up on the discussion in PR #2880 regarding simulator persistence and onboarding.

While the simulator provides a great "zero-config" experience, starting it without a user-provided JSON file often leads to confusion when clients attempt to request data from non-existent registers.

This PR adds a proactive warning in the entry point logic:

• Detects if the internal setup.json is being used as a fallback.
• Logs a WARNING indicating that a demo configuration is active.
• Provides explicit instructions on how to load a custom configuration using --json_file.

Technical Changes

• Modified pymodbus/server/simulator/main.py: Added logic check in get_commandline after logging initialization to ensure the warning is formatted correctly according to the user's log level.

Testing

-OS: Windows 11 Enterprise LTSC.

Shell: MINGW64 (Git Bash) / PowerShell Core.

• Running pymodbus.simulator now correctly triggers the warning logs.
• Running with a custom --json_file bypasses the warning as expected.

[janiversen]

LGTM,thanks.