Ensure PR description is not too long

[mzuehlke]

With the addition of structured metadata to PR description the content was growing too large when multiple updates got grouped together. Now the metadata gets only appended, if the length is still below 65536 characters.

Caused-by: #3466
Fixes: #3476

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.66%. Comparing base (dce40bb) to head (5b1fa0f).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...lasteward/core/forge/data/NewPullRequestData.scala	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3492      +/-   ##
==========================================
- Coverage   89.67%   89.66%   -0.02%     
==========================================
  Files         171      171              
  Lines        3526     3530       +4     
  Branches      357      332      -25     
==========================================
+ Hits         3162     3165       +3     
- Misses        364      365       +1     

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[alejandrohdezma]

LGTM