Fix OpenApiEncoding explode property serialization

[David Brodski (kammerjaeger)]

Fix: Explode serialization is wrong when style is "form"

This fixes the same problem as #544 but for OpenApiEncoding.
The spec describes different behavior depending on if it is set or not (see: https://spec.openapis.org/oas/v3.1.0.html#fixed-fields-12) and also different defaults depending on the style value.
This also needs to be cherry picked to main, I just need it for v1 but will create a pr to main with the change if this pr is fine.

Please let me know what you think.

[Vincent Biret (baywet)]

Thanks for the contribution!

[Vincent Biret (baywet)]

David Brodski (@kammerjaeger) can you please also sign the CLA? (see the bot comment)

[David Brodski (kammerjaeger)]

@microsoft-github-policy-service agree company="AMD"

[David Brodski (kammerjaeger)]

Thanks for the fast review. How often do you release new versions?

[Vincent Biret (baywet)]

We make releases as needed, as long as it's possible because this is a maintenance branch.
This process is supposed to be automated, but for some reason it got stuck because we already have a mainline release???
https://github.com/microsoft/OpenAPI.NET/actions/runs/17869041888/job/50818331357

Once we release main, I'll re-run it manually, please ping me if I forget :)

[Vincent Biret (baywet)]

David Brodski (@kammerjaeger) I had to trigger the release manually to circumvent the the issue, I'm waiting for somebody to approve it #2513