docs: unify crawl caps and fix the runnable code examples

[vdusek]

Fixes the "Run on Apify" examples in the Python SDK guides.

• Lower the page cap from 50 to 10 across all crawling examples, so the browser-based ones finish within the 180s runnable-demo timeout.
• Selenium: slim the runnable example to a plain crawler (the full version was too large to encode into the Run-on-Apify URL and failed with HTTP 414), and move the proxy-auth extension into the "Using Apify Proxy" section as a separate, non-runnable extension snippet.
• Browser Use: make it non-runnable (it needs an LLM API key the shared runner cannot provide), with a comment explaining why. Same note added to the Scrapy and Scrapling-browser examples.
• Keep both Scrapling examples runnable.
• Pydantic: log a readable validation summary and fail cleanly via Actor.fail instead of re-raising into a raw traceback.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.46%. Comparing base (53122e4) to head (8a854f2).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1002      +/-   ##
==========================================
- Coverage   90.51%   90.46%   -0.06%     
==========================================
  Files          49       49              
  Lines        3132     3135       +3     
==========================================
+ Hits         2835     2836       +1     
- Misses        297      299       +2     

Flag	Coverage Δ
e2e	36.10% <ø> (-0.04%)	⬇️
integration	57.06% <ø> (-0.06%)	⬇️
unit	79.23% <ø> (-0.05%)	⬇️

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