[python-fastapi] return 500 if not implemented, added some unittests

[krjakbrjak]

• return 500 if not implemented
• added some unittests

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cc: @cbornet (2017/09) @tomplus (2018/10) @krjakbrjak (2023/02) @fa0311 (2023/10) @multani (2023/10)

[wing328]

thanks for the PR, which has been merged.

i also updated the PR subject and description.