[python-fastapi] Use ... as default value rather than None (#22494)

[zhornback]

With various FastAPI parameters (Query, Path, Header, Cookie, Body, Form, and File) the default value of None is a valid value, but signifies that the parameter is optional. To allow for required parameters, default should either not be set at all or be set to '...' (ellipsis).

Reproduction details can be found in #22494.

fixes #22494

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---

Summary by cubic

Switch the python-fastapi generator to emit ... as the default for required params instead of None, so FastAPI correctly treats them as required. Fixes #22494 and aligns generated endpoints with OpenAPI “required” semantics across all param types.

• Bug Fixes
  • Update endpoint_argument_definition.mustache to use ... when required, otherwise None, for Query, Path, Header, Cookie, Body, Form, and File.
  • Regenerate samples to reflect required params using ... (e.g., pet/order/user bodies, status/tags/username/password queries).

^{Written for commit 63a2d8a. Summary will update on new commits.}

[cubic-dev-ai]

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