Ensure markdown parsing does not choke on partial links

[philippjfr]

Fixes #6520

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 82.87%. Comparing base (9641c9b) to head (e202069).
Report is 1 commits behind head on main.

Files	Patch %	Lines
panel/pane/markup.py	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6535      +/-   ##
==========================================
+ Coverage   81.86%   82.87%   +1.01%     
==========================================
  Files         313      313              
  Lines       45931    45945      +14     
==========================================
+ Hits        37600    38077     +477     
+ Misses       8331     7868     -463     

Flag	Coverage Δ
ui-tests	40.09% <26.66%> (+3.04%)	⬆️
unitexamples-tests	71.07% <73.33%> (-0.09%)	⬇️

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