Fixes #24360.

The circe branch of the scala-sttp4 generator uses plain deriveEncoder, which serializes Option None as JSON null:

implicit val encoder: Encoder[Pet] = deriveEncoder
// Pet("rex", None) => {"name":"rex","tag":null}

The scala-sttp (sttp3) generator omits absent optional fields — its hand-rolled encoders build fields via t.tag.map(v => "tag" -> v.asJson) — as do the json4s branch (json4s omits None by default) and scala-sttp4-jsoniter (transientNone defaults to true). So clients migrating sttp3 → sttp4 with circe silently start sending explicit nulls, which strict servers (e.g. FastAPI/pydantic non-nullable optional fields) reject with 422.

Fix: append .mapJson(_.dropNullValues) to the generated circe encoders — plain models, inline oneOf members, and the discriminator-configured sealed-trait encoder — restoring sttp3 parity: None ⇒ field omitted. The wrapper-composition oneOf encoders delegate to member encoders and inherit the fix. Verified end-to-end: with this change Pet(name = "rex", tag = None).asJson.noSpaces == {"name":"rex"} and the regenerated scala-sttp4-circe petstore sample compiles.

Known limitation (unchanged from scala-sttp): Option cannot express an explicit JSON null for nullable: true properties; distinguishing null-vs-absent would require a tri-state type and is out of scope here.

Tests: Sttp4CodegenTest#verifyOptionalFieldsOmittedWhenNone with a new fixture (sttp4-optional-fields.yaml). Samples regenerated via ./bin/generate-samples.sh bin/configs/scala-sttp4*.yaml — only the circe sample changes (json4s/jsoniter are unaffected, confirmed by zero-diff regeneration).

PR checklist

• Read the contribution guidelines.
• Built the project and updated samples; changes committed.
• Filed against master.
• Scala technical committee: @clasnake @jimschubert @shijinkui @ramzimaalej @chameleon82 @Bouillie

Summary by cubic

Omit optional fields set to None in scala-sttp4 circe models by dropping null values during encoding. This restores sttp3 behavior and prevents sending JSON nulls that strict servers reject.

• Bug Fixes
  • Appended .mapJson(_.dropNullValues) to all generated circe encoders (plain models, inline oneOf members, and discriminator-based sealed traits).
  • Added test verifyOptionalFieldsOmittedWhenNone with sttp4-optional-fields.yaml.
  • Regenerated scala-sttp4-circe petstore samples; other libraries (json4s, scala-sttp4-jsoniter) unchanged.

^{Written for commit 2a3769c. Summary will update on new commits.}