[chore][cmd/mdatagen] Require all fields to have a non-empty description

[mx-psi]

Description

Requires all fields to have a non-empty description.

I am keeping this as draft until we have some way of configure mdatagen.

Authorship

• I, a human, wrote this pull request description myself.

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.03%. Comparing base (52e6bf4) to head (1692a18).

Files with missing lines	Patch %	Lines
internal/schemagen/model.go	87.09%	2 Missing and 2 partials ⚠️

❌ Your patch check has failed because the patch coverage (88.23%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15654      +/-   ##
==========================================
- Coverage   91.03%   91.03%   -0.01%     
==========================================
  Files         729      729              
  Lines       48491    48525      +34     
==========================================
+ Hits        44144    44174      +30     
- Misses       3020     3022       +2     
- Partials     1327     1329       +2     

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