refactor: Use Pydantic models instead of custom dict representations

[vdusek]

Summary

• Replace hand-written _representations.py helper functions with Pydantic request models (CreateActorRequest, UpdateActorRequest, ScheduleCreate, WebhookCreate, etc.) for constructing API payloads
• Simplify _update() / _create() in ResourceClient to accept **kwargs and use a new _clean_json_payload() method instead of the removed filter_none_values() utility
• Remove unused enum_to_value() from _utils.py — Pydantic serialization handles enum conversion

Test plan

• Verify existing unit tests pass
• Verify integration tests pass against the Apify API
• Confirm no regressions in Actor, Task, Schedule, Webhook, and Version CRUD operations

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 57.41935% with 66 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.87%. Comparing base (230bb64) to head (1b03bbc).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/apify_client/_resource_clients/webhook.py	20.00%	8 Missing ⚠️
...ify_client/_resource_clients/webhook_collection.py	20.00%	8 Missing ⚠️
src/apify_client/_resource_clients/actor.py	42.85%	4 Missing ⚠️
...apify_client/_resource_clients/actor_collection.py	33.33%	4 Missing ⚠️
...rc/apify_client/_resource_clients/actor_version.py	42.85%	4 Missing ⚠️
...ient/_resource_clients/actor_version_collection.py	33.33%	4 Missing ⚠️
src/apify_client/_resource_clients/schedule.py	33.33%	4 Missing ⚠️
...fy_client/_resource_clients/schedule_collection.py	20.00%	4 Missing ⚠️
src/apify_client/_resource_clients/task.py	20.00%	4 Missing ⚠️
.../apify_client/_resource_clients/task_collection.py	33.33%	4 Missing ⚠️
... and 9 more

❗ There is a different number of reports uploaded between BASE (230bb64) and HEAD (1b03bbc). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (230bb64)	HEAD (1b03bbc)
integration	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #650       +/-   ##
===========================================
- Coverage   96.62%   76.87%   -19.76%     
===========================================
  Files          45       45               
  Lines        4271     4320       +49     
===========================================
- Hits         4127     3321      -806     
- Misses        144      999      +855     

Flag	Coverage Δ
integration	?
unit	76.87% <57.41%> (+1.27%)	⬆️

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