refactor: Use Pydantic models instead of custom dict representations

[vdusek]

Summary

• Replace hand-written _representations.py helper functions (302 lines deleted) with Pydantic request models (UpdateActorRequest, ScheduleCreate, WebhookCreate, WebhookUpdate, etc.) for constructing API payloads.
• Simplify _update() / _create() in ResourceClient to accept **kwargs and use a new _clean_json_payload() static method on ResourceClientBase instead of the removed filter_none_values() utility.
• Remove unused enum_to_value() and filter_none_values() from _utils.py — Pydantic serialization handles enum conversion and None filtering.
• Add populate_by_name=True to all generated Pydantic model configs via datamodel-codegen configuration, so models can be initialized using either snake_case field names or camelCase aliases.
• Models had to be adjusted, see PR to apify-docs - fix(openapi): Update OpenAPI schemas to match API behaviour apify-docs#2291.

Test plan

• CI passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (5ae33a0) to head (f2133a4).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #650      +/-   ##
==========================================
- Coverage   96.59%   96.52%   -0.07%     
==========================================
  Files          46       45       -1     
  Lines        4433     4318     -115     
==========================================
- Hits         4282     4168     -114     
+ Misses        151      150       -1     

Flag	Coverage Δ
integration	94.55% <100.00%> (-0.10%)	⬇️
unit	78.04% <80.90%> (+1.52%)	⬆️

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