tests: corrupt manifest off-centre to fix flaky check --repair rebuild test on Windows

[mr-raj12]

test_manifest_rebuild_corrupted_chunk corrupted the manifest at a fixed offset of 250 bytes. A repository layout change moved that byte outside the encrypted manifest data on Windows, so check --repair no longer hit the rebuild path and the test failed on the Windows Canary job. Corrupting at len(manifest)//3 puts the byte back inside the payload. The test passes locally for the local and remote variants.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.18%. Comparing base (2dcb50f) to head (26d3a5d).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9827      +/-   ##
==========================================
- Coverage   85.23%   85.18%   -0.05%     
==========================================
  Files          92       92              
  Lines       15297    15303       +6     
  Branches     2299     2300       +1     
==========================================
- Hits        13038    13036       -2     
- Misses       1570     1578       +8     
  Partials      689      689              

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