Ensure root_url is correctly determined during auth

[philippjfr]

• Fixes Invalid azure oauth redirect when using prefix #7676
• Add tests (relies on Configure proxy tests #7679)

[codecov]

Codecov Report

Attention: Patch coverage is 24.79339% with 91 lines in your changes missing coverage. Please review.

Project coverage is 86.04%. Comparing base (c2c2d9c) to head (071860a).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/tests/ui/test_auth.py	23.28%	56 Missing ⚠️
panel/auth.py	8.33%	22 Missing ⚠️
panel/command/serve.py	22.22%	7 Missing ⚠️
panel/tests/util.py	60.00%	4 Missing ⚠️
panel/io/server.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7680      +/-   ##
==========================================
- Coverage   86.63%   86.04%   -0.60%     
==========================================
  Files         346      346              
  Lines       52530    52625      +95     
==========================================
- Hits        45510    45280     -230     
- Misses       7020     7345     +325     

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