fix(python): reduce flaky wire tests

[kafkas]

Description

Linear ticket: FER-8170

Fixes multiple issues that caused Python wire tests and snippets to be invalid or fail at runtime. It improves wire test setup and makes snippet/request encoding resilient to omitted/missing example values. Together, these changes unblock generating and running wire tests across a wider variety of SDK configurations.

Changes Made

• Handle the Ellipsis OMIT sentinel in JSON encoding to prevent 'ellipsis' object is not iterable errors and avoid serializing omitted values.
• Respect inline_request_params in snippet generation so request bodies can be passed correctly without unexpected keyword args.
• Fix undiscriminated union snippet mapping by skipping nop() instantiations when selecting a matching union member.
• Prefer OK examples / avoid asserting error paths when stubs aren’t generated to prevent false failures like “DID NOT RAISE ApiError”.
• Synthesize placeholder values for required multipart file fields when examples omit them so file-upload endpoints don’t fail with missing required args.

Testing

• Unit tests added/updated
• Manual testing completed

[github-actions]

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