Fix: don't issue tasks for parent schema definition documents in schema task

[sandreae]

📋 Checklist

• Add tests that cover your changes
• Add this PR to the Unreleased section in CHANGELOG.md
• Link this PR to any issues it closes
• New files contain a SPDX license header

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (a24b2d8) 92.46% compared to head (8e72b5e) 92.69%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #581      +/-   ##
==========================================
+ Coverage   92.46%   92.69%   +0.23%     
==========================================
  Files         106      106              
  Lines       18400    18384      -16     
==========================================
+ Hits        17013    17041      +28     
+ Misses       1387     1343      -44     

Files	Coverage Δ
aquadoggo/src/materializer/tasks/dependency.rs	97.39% <100.00%> (-0.14%)	⬇️
aquadoggo/src/materializer/tasks/schema.rs	96.26% <97.18%> (+8.03%)	⬆️
aquadoggo/src/materializer/tasks/blob.rs	84.25% <84.61%> (+28.05%)	⬆️

... and 2 files with indirect coverage changes

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