test(i): Fix debug explain test-framework with similarity

[shahzadlone]

Relevant issue(s)

Resolves #3471

Description

• Add compile time check to ensure similarity node is explainable.
• Fix debug explain test-framework with similarity query
• Add a debug explain test

Tasks

• I made sure the code is well commented, particularly hard-to-understand areas.
• I made sure the repository-held documentation is changed accordingly.
• I made sure the pull request title adheres to the conventional commit style (the subset used in the project can be found in tools/configs/chglog/config.yml).
• I made sure to discuss its limitations such as threats to validity, vulnerability to mistake and misuse, robustness to invalidation of assumptions, resource requirements, ...

How has this been tested?

CI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.39%. Comparing base (05f9fb1) to head (2f62a5a).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3473      +/-   ##
===========================================
+ Coverage    78.37%   78.39%   +0.02%     
===========================================
  Files          397      397              
  Lines        37571    37571              
===========================================
+ Hits         29445    29451       +6     
+ Misses        6423     6418       -5     
+ Partials      1703     1702       -1     

Flag	Coverage Δ
all-tests	78.39% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
internal/planner/explain.go	59.40% <ø> (ø)

... and 7 files with indirect coverage changes

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[fredcarle]

LGTM