Cover Scala 3 core gaps (formats, Seq, empty struct, summoner)

[JavierMonton]

Scala 3 coverage jobs measure core's scala_3 sources for the first time and report uncovered lines there (local core statement coverage was 75.58%). This adds the missing tests in the existing AnyFlatSpec/Matchers style: a FormatsSpec (default/snakify/key-type transforms), plus Seq conversion, empty-struct derivation and the SqlInstanceConversion summoner. Local core coverage on 3.3.3 is now 100% statement and branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.33%. Comparing base (0b39dc6) to head (e89eec0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #661      +/-   ##
==========================================
+ Coverage   88.77%   91.33%   +2.55%     
==========================================
  Files          32       32              
  Lines         508      508              
  Branches       55       62       +7     
==========================================
+ Hits          451      464      +13     
+ Misses         57       44      -13     

Flag	Coverage Δ
unittests	85.82% <ø> (+2.55%)	⬆️

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