Don't use sjson.GetTypeOfValue where it shouldn't be used

[noisersup]

Description

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[AlekSi]

They happen to do the same thing.

sjson layer is responsible for preserving BSON-type information in storage. Currently, we store type information in the special schema field. For example, for "64-bit binary floating point" value, we store a string "double". That string could be replaced with a number or even a bitmask as long as we preserve backward compatibility for client applications (and not even that if we bump a major version).

Type aliases are fixed and cannot be changed without breaking compatibility with MongoDB.

What we need to do instead is:

• make this difference clear from code doc comments;
• check that we don't use a wrong function somewhere (spoiler: we do at least in once place)

[codecov]

Codecov Report

Merging #2728 (04b41c2) into main (978278c) will decrease coverage by 36.85%.
The diff coverage is 0.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2728       +/-   ##
===========================================
- Coverage   63.04%   26.19%   -36.85%     
===========================================
  Files         441      441               
  Lines       22619    22619               
===========================================
- Hits        14260     5926     -8334     
- Misses       7434    16087     +8653     
+ Partials      925      606      -319     

Impacted Files	Coverage Δ
internal/handlers/commonparams/typecode.go	48.42% <ø> (-43.16%)	⬇️
internal/handlers/pg/msg_dropindexes.go	0.00% <0.00%> (-88.10%)	⬇️
internal/handlers/sjson/schema.go	83.78% <ø> (ø)

... and 143 files with indirect coverage changes

Flag	Coverage Δ
integration	4.96% <0.00%> (-51.54%)	⬇️
mongodb	4.96% <0.00%> (ø)
pg	?
unit	24.66% <0.00%> (ø)

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[rumyantseva]

👌