fix(scrapy): dump pydantic models in JSON mode when serializing requests

[vdusek]

Pydantic models in a Scrapy request's meta/cb_kwargs were dumped in python mode, so any model with a non-JSON-native field (datetime, UUID, HttpUrl, ...) failed json.dumps and the request was dropped. The v4 upgrade guide promises such models keep working.

• Dump models with model_dump(mode='json', by_alias=True).
• Widen the serialization error guards to except (TypeError, ValueError) to cover pydantic's PydanticSerializationError.
• Add a round-trip test with a datetime-bearing model and update the v4 upgrade guide wording.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.94%. Comparing base (a87e8d1) to head (140fff9).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #961      +/-   ##
==========================================
- Coverage   90.00%   89.94%   -0.07%     
==========================================
  Files          49       49              
  Lines        3053     3053              
==========================================
- Hits         2748     2746       -2     
- Misses        305      307       +2     

Flag	Coverage Δ
e2e	35.66% <0.00%> (ø)
integration	56.86% <0.00%> (ø)
unit	78.64% <100.00%> (-0.07%)	⬇️

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.