refactor: unify alias vs validation_alias usage in Pydantic models

[vdusek]

Pydantic's alias sets both the validation and the serialization name, while validation_alias affects only validation. Configuration mixed the two, so model_dump(by_alias=True) was incoherent — e.g. is_at_home dumped as apify_is_at_home but started_at dumped as started_at.

This makes serialization consistent: model_dump(by_alias=True) now emits camelCase names derived from the Python field name across all Pydantic models.

• _configuration.py — each field is validated from its environment variable(s) via validation_alias (a plain string, or AliasChoices(...) for legacy names). Serialization is handled by a model-level alias_generator=AliasGenerator(serialization_alias=to_camel), so e.g. is_at_home → isAtHome, actor_id → actorId, max_paid_dataset_items → maxPaidDatasetItems. This also covers the fields inherited from Crawlee's base Configuration.
• API / event / data models (_charging.py, events/_types.py, storage_clients/_apify/_models.py, request_loaders/_apify_request_list.py) already serialize camelCase via alias_generator=to_camel; RequestQueueHead.lock_time keeps its explicit serialization_alias='lockSecs' where the wire name differs from the field-derived one. Left as-is.
• CLAUDE.md — documents the convention.

One inherited Crawlee field, internal_timeout, is defined with alias='crawlee_internal_timeout' (not validation_alias), which pins its serialization so the generator can't override it. It's outside this repo's control.

Closes #807

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.69%. Comparing base (4ead0c6) to head (1fb0cbc).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1022      +/-   ##
==========================================
- Coverage   91.72%   91.69%   -0.03%     
==========================================
  Files          50       50              
  Lines        3213     3215       +2     
==========================================
+ Hits         2947     2948       +1     
- Misses        266      267       +1     

Flag	Coverage Δ
e2e	35.64% <100.00%> (+0.04%)	⬆️
integration	57.38% <100.00%> (+0.02%)	⬆️
unit	83.01% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.