remove duplicated file name access and use StringBuilder to insert in WebServerPortFileWriter

[choi-hyeseong]

The File.getName() method always slice the path to get the name. so, it's not cached
It doesn't have a huge impact, but it's not efficient, so i fixed it :)

Also, the original code was truncating the extension from the filename, adding the namespace, and then putting the extension back together again, but i modified the code to be cleaner by using StringBuilder to do the insertion.

This change passed the test (WebServerPortFileWriterTests.java)

• Remove duplicated File.getName() method
• using StringBuilder to insert namespace

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[philwebb]

Thanks very much @choi-hyeseong, I wasn't aware that calls to getName() aren't cached. I'm not totally sure about using a StringBuilder here, to me it makes things a little harder to read. I've changed things back to using Strings, but tried to improve the variable names a little in 60f6158.

[choi-hyeseong]

Thank you for considering my PR :)