Automatic port for manual tests.

[przemyslaw-zan]

🚀 Summary

The manual test server now automatically finds a free port at startup. When the preferred port (default 8125) is already in use, the server tries subsequent ports until an available one is found. The --port option can be used to set the starting port. The server writes the selected port to a .port file in the build directory so that external scripts can discover which port is in use.

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📌 Related issues

• Closes https://github.com/ckeditor/ckeditor5-internal/issues/4352.

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💡 Additional information

Testing: Block the port 8125 with any server, then launch manual tests. They should start at 8126.

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