[Backend.Tests] Change sealed internal to internal sealed

[imnasnainaec]

Match our usage of public sealed and private sealed.
Also remove unnecessary empty exception constructors.

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.92%. Comparing base (8c6b909) to head (3066e9c).
⚠️ Report is 39 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3833   +/-   ##
=======================================
  Coverage   72.92%   72.92%           
=======================================
  Files         286      286           
  Lines       10771    10768    -3     
  Branches     1340     1340           
=======================================
- Hits         7855     7853    -2     
+ Misses       2516     2515    -1     
  Partials      400      400           

Flag	Coverage Δ
backend	81.92% <ø> (+<0.01%)	⬆️
frontend	65.87% <ø> (ø)

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[myieye]

LGTM 😄

Reviewed 20 of 20 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)