Add insert documents type validation

[w84thesun]

Description

Found this while working on reimplementing delete command.

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.

[codecov]

Codecov Report

Merging #2946 (837bf20) into main (11a2389) will decrease coverage by 4.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2946      +/-   ##
==========================================
- Coverage   63.96%   59.97%   -4.00%     
==========================================
  Files         451      451              
  Lines       24070    24074       +4     
==========================================
- Hits        15397    14438     -959     
- Misses       7712     8646     +934     
- Partials      961      990      +29     

Impacted Files	Coverage Δ
internal/handlers/common/insert.go	100.00% <100.00%> (ø)
internal/handlers/pg/msg_insert.go	88.42% <100.00%> (+4.76%)	⬆️

... and 48 files with indirect coverage changes

Flag	Coverage Δ
integration	53.01% <100.00%> (-4.39%)	⬇️
mongodb	4.91% <0.00%> (+0.07%)	⬆️
pg	52.80% <100.00%> (-4.53%)	⬇️
shard-1	4.91% <0.00%> (-36.87%)	⬇️
shard-2	42.31% <100.00%> (+0.12%)	⬆️
shard-3	46.03% <40.00%> (-0.01%)	⬇️
unit	24.13% <0.00%> (+0.01%)	⬆️

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

[rumyantseva]

Nice catch!

[noisersup]

Nice catch