ci: add workflow to regenerate models from OpenAPI spec (#694)

## Summary

- Add `workflow_dispatch` workflow that regenerates Pydantic models from
an `apify-docs` PR's OpenAPI spec
- Builds the spec locally, runs `datamodel-codegen`, and opens a PR if
models changed
- Comments back on the source `apify-docs` PR with a link

## Issues

- Closes: https://github.com/apify/apify-client-python/issues/618

## Context

- https://github.com/apify/apify-docs/pull/2381
- https://github.com/apify/apify-client-python/pull/694

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>