create the credential cache directory with 0700 in JSONFileCache

[ubeddulla]

JSONFileCache creates its working directory with os.makedirs and no mode, so it lands at 0o777 & ~umask. That directory backs ~/.aws/sso/cache (SSO access and refresh tokens) and ~/.aws/boto/cache (cached assume-role credentials); the entries themselves are already 0600 since they come from mkstemp and os.replace keeps the mode, but under umask 000, which is easy to hit in containers and in systemd units with no UMask=, the directory is world-writable and another local user can unlink an entry and drop in their own for botocore to load back. Passing mode=0o700 lines this up with what the CLI does for the directories it creates, and I added a regression test alongside the existing atomic-write ones.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.42%. Comparing base (ee748e4) to head (3bd1533).
⚠️ Report is 381 commits behind head on develop.
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3753      +/-   ##
===========================================
- Coverage    92.59%   92.42%   -0.18%     
===========================================
  Files           68       68              
  Lines        15634    15829     +195     
===========================================
+ Hits         14477    14630     +153     
- Misses        1157     1199      +42     

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[ubeddulla]

gentle ping