oauth2: replace a magic number with AuthStyleUnknown

[masonelmore]

I was recently looking through this code while troubleshooting an OAuth configuration. The magic number "0" wasn't immediately clear me to me until I read through the rest of the code. I think this change will make it easier to understand for people new to the codebase.

The AuthStyleUnknown constant can be found on line 113 in internal/token.go.

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