[python-fastapi] Fix anyOf schema deserialization / serialization and fix two small other defects

[AntoineDuComptoirDesPharmacies]

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

This pull request main objective is to fix the issue #19453

• Fix a bug of deserializing / serializing anyOf schemas
• Fix a bug where prefix was not extracted from OpenApi file
• Fix a bug which URL query param default string value having ':' special character

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