ci: Add nightly model regeneration from the published OpenAPI spec

[vdusek]

Replaces the apify-docs-triggered model regeneration with a nightly job, and records which published OpenAPI specification the generated models came from.

What changes

• on_schedule_regenerate_models.yaml - nightly at 02:00 UTC plus manual dispatch. Regenerates on master and opens a pull request when the models change. manual_regenerate_models.yaml is deleted.
• scripts/openapi_spec.py - downloads and validates the published specification into git-ignored tmp/ (both codegen passes read that one copy), then records its info.version in pyproject.toml under [tool.apify.openapi-spec]. The specification itself is not committed.
• _models.py loses the Plan.available_proxy_groups docstring - accumulated codegen drift from the datamodel-code-generator bumps (chore(deps): lock file maintenance #968, chore(deps): lock file maintenance #975), not a specification change.

Why

The old workflow was dispatched by apify-docs and checked out a permanent, never-rebased branch before regenerating, so it generated with that branch's stale tooling. In #979 that produced _literals.py in the pre-#941 closed Literal[...] form; merging it would have silently reverted the enum relaxation. It also opened with a TODO title that blocked pr-title-check - a design that needs human action to become mergeable, which is what let #979 rot for 20 days while the same spec changes were landed by hand (#923, #936, #947, #960, #974).

The nightly job always generates on master and rebuilds its ci/regenerate-models branch from master instead of appending, so the diff is always "current spec vs current master" and can't resurrect a stale generated file. It opens with a mergeable chore: title; reviewers retitle to fix:/feat: when the diff is user-facing.

Notes

• Merge ci: Drop cross-repo model regeneration dispatch to apify-client-python apify-docs#2835 first - it removes the dispatch that still calls the workflow deleted here.
• Needs a SLACK_WEBHOOK_URL repository secret for the failure alert.
• The recorded version is a coarse marker, not a content identity: apify-docs bumps components/version.yaml in a follow-up [skip ci] commit, so a deploy can publish new content under the old stamp. A moved value proves the specification changed; an unchanged one proves nothing.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.63%. Comparing base (cb8fe24) to head (85e396c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #983      +/-   ##
==========================================
- Coverage   94.65%   94.63%   -0.03%     
==========================================
  Files          58       58              
  Lines        5259     5258       -1     
==========================================
- Hits         4978     4976       -2     
- Misses        281      282       +1     

Flag	Coverage Δ
integration	92.12% <ø> (-0.31%)	⬇️
unit	84.04% <ø> (-0.01%)	⬇️

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[janbuchar]

Looks sound at a glance, let's battle test it