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 95.47%. Comparing base (53a9954) to head (f77e27b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1141      +/-   ##
==========================================
+ Coverage   95.43%   95.47%   +0.03%     
==========================================
  Files          61       61              
  Lines        5935     5940       +5     
==========================================
+ Hits         5664     5671       +7     
+ Misses        271      269       -2     

Flag	Coverage Δ
integration	90.65% <90.90%> (-0.15%)	⬇️
unit	88.88% <100.00%> (+<0.01%)	⬆️

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[Mantisus]

LGTM

[apify-approval-mirror]

Mirroring approval from @Mantisus.