Relax enum validation in generated models to tolerate unknown/new API values

[vdusek]

Enum fields in the generated models are strict Literal[...] aliases (src/apify_client/_literals.py).
Since every API response is validated with Pydantic, any enum value the platform sends that the
(older) client doesn't know raises a ValidationError and the whole call fails.

This means every new backend enum value breaks clients in the wild until we regenerate and release.
See apify/apify-sdk-python#1032 (missing MCP origin) and the reactive patch #929.

Proposal: emit an open literal Literal[...] | str instead of bare Literal[...] in the
postprocess step (scripts/postprocess_generated_models.py), so unknown values validate while known
members stay as autocomplete hints.