[Scala sttp] Fix header serialization for Optional values

[Mattias-Sehlstedt]

fixes #21602

Marked as a draft while attempting to find a proper solution for how to handle required header parameters that are complex (i.e., the header contains a ref rather than a string).

paths:
  /ping:
    get:
      summary: Ping with required header
      operationId: getPing
      parameters:
        - name: X-Required-Schema-Header
          in: header
          required: true
          schema:
            $ref: '#/components/schemas/UUID'
components:
  schemas:
    UUID:
      type: object
      properties:
        uuid:
          type: string

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

how to handle required header parameters that are complex (i.e., the header contains a ref rather than a string).

meanwhile we can consider that as an edge case and only start looking into it if users have such a use case

[wing328]

is this ready for merge?

if yes, i'll try to get it merged over the weekend after running some local tests

[Mattias-Sehlstedt]

Hi, I would say it is ready for merging give the small size of the changes.

I had hoped that I would get additional input from the issue author, but not something I have received yet.