smoke_test: Check failed publish with large payload

[Turbo87]

The "invalid authentication" check (see #10104) was actually resulting in a false-positive, since the request payload was small enough to not trigger the Cloudfront bug. This commit adjusts the smoke test to generate a 15 MB dummy file which we then try to publish instead.

Related:

• 503 on publishing a crate #10098
• smoke_test: Add "publish with invalid authentication" check #10104

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 61 lines in your changes missing coverage. Please review.

Project coverage is 89.37%. Comparing base (e6ec730) to head (e0afe0c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/crates_io_smoke_test/src/main.rs	0.00%	61 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10106      +/-   ##
==========================================
- Coverage   89.45%   89.37%   -0.08%     
==========================================
  Files         294      294              
  Lines       31256    31283      +27     
==========================================
+ Hits        27959    27960       +1     
- Misses       3297     3323      +26     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.