Add QueryContext deserialization test

[villebro]

CATEGORY

Choose one

• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• Documentation

SUMMARY

Fix a few bugs discovered during testing and add unit test that would have caught the deserialization error.

TEST PLAN

Local testing + CI

ADDITIONAL INFORMATION

• Has associated issue: [SIP-38] Visualization plugin refactoring #9187
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #9778 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #9778   +/-   ##
=======================================
  Coverage   70.77%   70.77%           
=======================================
  Files         587      587           
  Lines       30460    30460           
  Branches     3121     3121           
=======================================
  Hits        21558    21558           
  Misses       8788     8788           
  Partials      114      114           

Flag	Coverage Δ
#cypress	53.66% <ø> (ø)
#javascript	59.00% <ø> (ø)
#python	70.93% <100.00%> (ø)

Impacted Files	Coverage Δ
superset/charts/schemas.py	100.00% <100.00%> (ø)
superset-frontend/src/SqlLab/actions/sqlLab.js	66.81% <0.00%> (ø)

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