Extract text from annotated shapes and use it in thumbnail alt text

[robertknight]

When the area selected for a rect or pin annotations contains PDF text, extract that text and store it in the shape selector. This is then used to generate better alt text for the thumbnail in the sidebar:

The length of the extracted text is capped to stop the selector JSON from becoming too large.

In future we plan to enable users to enter a text description for the thumbnail. That description will either replace or be combined with the extracted text.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.39%. Comparing base (bbed920) to head (0a79f88).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7077   +/-   ##
=======================================
  Coverage   99.38%   99.39%           
=======================================
  Files         277      278    +1     
  Lines       11273    11314   +41     
  Branches     2719     2727    +8     
=======================================
+ Hits        11204    11245   +41     
  Misses         69       69           

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