ci: fix multipart regression lint failures

[etr]

The raw multipart upload regression helper introduced three cpplint runtime/int failures, leaving Verify Build red. Add scoped annotations for the long types required by libcurl's POSTFIELDSIZE and response-code APIs, with a rationale matching the existing upload helpers.

Validation: reproduced the three CI failures twice before the change; the full recursive cpplint command now passes. Warning-suppression and file-size gates pass, and git diff --check is clean. This changes only comments and ChangeLog; executable test behavior is unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.51%. Comparing base (ae84459) to head (4a12d5d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
- Coverage   69.85%   69.51%   -0.34%     
==========================================
  Files          80       80              
  Lines        4229     4229              
  Branches     1521     1521              
==========================================
- Hits         2954     2940      -14     
- Misses        366      372       +6     
- Partials      909      917       +8     

see 13 files with indirect coverage changes

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