Fix flaky storage progress tests, bump AWSSDK.S3 to 4.0.103.4

[Menelion]

Summary

• Flaky test fix: SftpStorageTests.ProgressChanged_LargeFile_RaisesEvents failed intermittently on Ubuntu CI (Assert.All() Failure: 1 out of 480 items… Item: null). SSH.NET invokes the upload progress callback from multiple threads, and the test collected events in a plain List<T>, so concurrent Add calls occasionally left a null slot. The SFTP, FTP (FluentFTP's Progress<T> posts to the thread pool) and S3 (parallel multipart parts) progress tests now use ConcurrentQueue<T>. WebDAV's ProgressStream is sequential and is unchanged. Test-only change; no library code touched.
• Dependency: AWSSDK.S3 4.0.103.1 → 4.0.103.4, superseding the closed Deps: Bump AWSSDK.S3 from 4.0.103.1 to 4.0.103.4 #124 (whose CI failure was this flaky test).

Note

LocalStack is currently not starting in CI: localstack/localstack:latest now requires LOCALSTACK_AUTH_TOKEN ("License activation failed… No credentials were found"), so the S3 integration tests are skipped. Fixing that needs either the secret (as both an Actions and a Dependabot secret) or pinning an older LocalStack image; not addressed here.

Test plan

• dotnet build: 0 warnings, 0 errors
• dotnet format --verify-no-changes: clean
• dotnet test locally: 819 passed, 130 integration tests skipped
• CI green on Ubuntu/Windows/macOS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.82%. Comparing base (05b860c) to head (df67113).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #126   +/-   ##
=======================================
  Coverage   78.82%   78.82%           
=======================================
  Files          44       44           
  Lines        4874     4874           
  Branches      726      726           
=======================================
  Hits         3842     3842           
  Misses        775      775           
  Partials      257      257           

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