fix: raise a helpful error for pre-v4 ActorEventTypes enum-member access

[vdusek]

apify.ActorEventTypes became a bare Literal[...] alias in v4 (it was a StrEnum in v3). Since a typing.Literal object has no attributes, pre-v4 code doing ActorEventTypes.SYSTEM_INFO failed with a cryptic AttributeError: SYSTEM_INFO that gave no pointer to the migration.

This keeps the Literal[...] alias for type checkers and annotations, but at runtime binds ActorEventTypes to a shim whose __getattr__ raises a helpful AttributeError pointing at apify.Event.<NAME> and the v4 upgrade guide. Legitimate type-annotation use (e.g. event_type: ActorEventTypes) is unaffected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.70%. Comparing base (ed1d8eb) to head (90aa47e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1050      +/-   ##
==========================================
- Coverage   91.75%   91.70%   -0.06%     
==========================================
  Files          50       50              
  Lines        3215     3219       +4     
==========================================
+ Hits         2950     2952       +2     
- Misses        265      267       +2     

Flag	Coverage Δ
e2e	?
integration	57.34% <57.14%> (-0.05%)	⬇️
unit	83.03% <100.00%> (-0.05%)	⬇️

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