Fix manifest file escaping in URLs

[Swiddis]

Description

Adds a reproducer test for #4656 based on my best understanding of the inputs that led to the bug, and updates the code to fix it with more robust join logic. Some other whitespace updates got mangled in due to autoformatting, can filter them back out if necessary.

A cleaner alternative may be to go back to using os.path.join and replacing \\ with /, but could still run into issues if the input strings already have backslashes. urllib.parse.urljoin frustratingly does not actually join URLs (it cuts away path components), so as far as I can tell using .strip('/') everywhere is the best way to merge URLs.

Issues Resolved

Resolves #4656

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.03%. Comparing base (46f773e) to head (6ca1963).
Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4660      +/-   ##
==========================================
- Coverage   92.40%   92.03%   -0.37%     
==========================================
  Files         193      193              
  Lines        6317     6350      +33     
==========================================
+ Hits         5837     5844       +7     
- Misses        480      506      +26     

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

[zelinh]

LGTM.