Add support for Undefined BSON type

[AlekSi]

Closes FerretDB/FerretDB#4729.

Copilot reviewed 5 out of 9 changed files in this pull request and generated no comments.

Files not reviewed (4)

• wirebson/testdata/all.hex: Language not supported
• wirebson/decode.go: Evaluated as low risk
• wirebson/encode.go: Evaluated as low risk
• wirebson/size.go: Evaluated as low risk

Comments suppressed due to low confidence (1)

wirebson/logging.go:114

• Returning an empty slog.Value for UndefinedType might be ambiguous. Consider returning a more meaningful value.

return slog.Value{}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.38%. Comparing base (6f61731) to head (e116619).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   63.08%   63.38%   +0.29%     
==========================================
  Files          41       41              
  Lines        2395     2409      +14     
==========================================
+ Hits         1511     1527      +16     
+ Misses        700      698       -2     
  Partials      184      184              

Files with missing lines	Coverage Δ
wirebson/bson.go	82.85% <100.00%> (+0.63%)	⬆️
wirebson/decode.go	92.20% <100.00%> (+2.59%)	⬆️
wirebson/encode.go	61.76% <100.00%> (+0.86%)	⬆️
wirebson/logging.go	92.44% <100.00%> (+0.13%)	⬆️
wirebson/size.go	96.42% <100.00%> (+0.13%)	⬆️

Flag	Coverage Δ
test	63.38% <100.00%> (+0.29%)	⬆️

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[chilagrow]

This will make fuzzing happy 👍