fix(r2-downloader): reuse client for byte-range reads

[bhimrazy]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fixes R2 byte-range downloads re-creating the client on every call by checking _client, the attribute actually used by the downloader.

PR review

Anyone in the community is free to review the PR once the tests have passed.

Did you have fun?

Yes.

[bhimrazy]

requires #841

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81%. Comparing base (9650501) to head (2f730cc).
⚠️ Report is 1 commits behind head on main.
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Additional details and impacted files

@@         Coverage Diff         @@
##           main   #840   +/-   ##
===================================
  Coverage    81%    81%           
===================================
  Files        54     54           
  Lines      7671   7671           
===================================
+ Hits       6190   6198    +8     
+ Misses     1481   1473    -8     

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