Fix protocol-relative viewer file URLs

[mjibulu]

Summary

Resolve protocol-relative file parameters against the viewer location.

Problem

A value such as //host:port/document.pdf falls through to the fallback encoder, which percent-encodes the colon in the authority and prevents the PDF from loading.

Solution

Supply the viewer location as the URL base only for protocol-relative values. Absolute and ordinary relative URL handling remains unchanged.

Testing

• Added integration coverage for a protocol-relative PDF URL
• Confirmed the focused test fails before the fix
• Confirmed the focused test passes after the fix
• npx gulp lint
• npx gulp typestest
• npx gulp generic
• Relevant unit/integration results recorded above

Fixes #20218

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.00%. Comparing base (7fc7072) to head (e2c8def).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21676   +/-   ##
=======================================
  Coverage   90.00%   90.00%           
=======================================
  Files         263      263           
  Lines       66854    66854           
=======================================
+ Hits        60173    60174    +1     
+ Misses       6681     6680    -1     

Flag	Coverage Δ
browsertest	66.53% <ø> (-0.01%)	⬇️
integrationtest	69.36% <100.00%> (+<0.01%)	⬆️
unittest	57.83% <ø> (-0.01%)	⬇️
unittestcli	56.56% <ø> (+<0.01%)	⬆️

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[timvandermeij]

Closing since this got fixed via #21718 instead. Thank you for your contribution though!