fix: append /v2 to api_url only when it is missing

[vdusek]

ApifyClient(api_url='https://api.apify.com/v2') sent every request to /v2/v2/.... Both clients now append the /v2 segment to api_url and api_public_url only when the URL path doesn't already end with it, the same as the JS client.

Closes: #1138

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.51%. Comparing base (ecccadb) to head (59aa423).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1141   +/-   ##
=======================================
  Coverage   95.50%   95.51%           
=======================================
  Files          61       61           
  Lines        5942     5947    +5     
=======================================
+ Hits         5675     5680    +5     
  Misses        267      267           

Flag	Coverage Δ
integration	90.61% <90.90%> (-0.01%)	⬇️
unit	88.96% <100.00%> (+<0.01%)	⬆️

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[Mantisus]

LGTM

[apify-approval-mirror]

Mirroring approval from @Mantisus.