Add support for discriminator in OpenAPI v3

[lukasab]

Describe the PR
Adds a new @Discriminator route-handler annotation for OpenAPI v3. When a response (or request body) uses oneOf polymorphism (multiple @Success annotations with the same status code), a discriminator object can now be declared to tell clients which concrete type to use at runtime.

Relation issue
#2150 #829

Additional context
Builds on the existing oneOf support introduced in #1870 and extended in #2059. The discriminator should only be applied to schemas that already contain a oneOf composition, as per the OpenAPI 3.0 spec.

Implementation notes

• Follows the same deferred-processing pattern as @Produce
• discriminatorInfo *parsedDiscriminator is stored on OperationV3 to bridge the two phases. Open to feedback on whether there's a better place for this — it felt slightly out of place on the struct but couldn't find a cleaner alternative given the parse ordering constraints.
• Declaring @Discriminator twice on the same operation returns an error rather than silently overwriting.
• Applied to both responses and request body content.