Minimal API unions support in RDF and RDG as endpoint parameters and return types

[DeagleGross]

Current PR validates aspnetcore correctly integrates with System.Text.Json to support unions. Note, that this PR covers unions only as body-parameters (input) and as return types (output). Unions support for using them in query/route/header (basically non-body) will be covered in #66648.

There are several bugs/innacuracies/behavioral questions I observed in both aspnetcore/runtime which are now being tracked:

1. [Runtime] JsonSerializer throws when serializing null value in a value-type nullable union case runtime#128688
2. [Runtime] STJ: union vs. class asymmetry — missing JSON silently produces default(union) that crashes on serialize runtime#128834
3. [Aspnetcore] RequestDelegateGenerator: [AsParameters] emits unqualified type names — CS0234 when user namespace collides #66950
4. [Aspnetcore] Source generator silently drops [FromBody(EmptyBodyBehavior=Allow)] when two endpoints share a delegate signature #66912

Here is the list of scenarios I validated. Each row runs in both `CompileTimeCreationTests` (RDG / source-gen path) and `RuntimeCreationTests` (RDF / reflection path):

Request body binding

Scenario	Verified
Unambiguous primitive cases (bool/string, etc.)	Round-trip
Ambiguous primitive cases without classifier	400 BadRequest
Ambiguous primitive cases with classifier	Round-trip
Nullable union wrapper (Union?)	Value + null
Union with nullable case	Fails without classifier, passes with
Object-case union	Fails without classifier, passes with
Object-case union, payload of derived type	Resolves to nearest declared case
Nested union (union inside union)	Requires outer classifier
Ambiguous numeric union	Fails without classifier, passes with
Polymorphic case union ([JsonPolymorphic] cases)	Binds active case
ConfigureHttpJsonOptions honored	Yes
Explicit [FromBody]	Behaves like implicit body
Empty body	Status depends on nullability + policy
Non-JSON content type	415 UnsupportedMediaType
Union as property of wrapper DTO	Deserializes correctly
Union as property with [JsonRequired]	400 on missing, 400 on null, 200 on present
[AsParameters] container with union body + route slot	Union → body, sibling → route

Return types

Scenario	Verified
Primitive cases	Serialize as expected
Task<Union> / ValueTask<Union>	Serialize active case
Union? return	Value + null
Union with nullable case	Each case handled
Object-case union	Serializes active case
Nested union	Serializes innermost case
Runtime-type dispatch	Resolves to nearest declared case
Ambiguous numeric union	Serializes by .NET runtime type
Ambiguous string union	Serializes by .NET runtime type
Returned via Results<T1,T2> wrappers	Serializes active case
Union as property of wrapper DTO	Serializes active case
Union with polymorphic case type	Emits discriminator
Custom JsonTypeClassifier	Does not affect serialization
IAsyncEnumerable<Union>	Streams active case per item
ConfigureHttpJsonOptions honored	Yes

Closes #66542
Closes #66543

[Copilot]

Pull request overview

Adds Minimal API coverage for C# union types (System.Text.Json union support) across both reflection-based (RDF) and source-generated (RDG) request delegate creation paths, focusing on JSON body binding and JSON responses. Also updates the minimal sample to demonstrate union usage with JsonUnion classifiers.

Changes:

• Add a shared set of union test types (primitive unions, ambiguous unions, nullable cases, object-case unions, nested unions, polymorphic-case unions, classifier-backed unions).
• Add RDG/RDF tests validating union request-body binding behaviors (success, ambiguity → 400, classifier resolution, empty-body behaviors, non-JSON content-type → 415, wrapper DTO scenarios, [AsParameters]).
• Add RDG/RDF tests validating union response serialization behaviors (sync/Task/ValueTask, nullable wrapper, nested unions, ambiguous unions, runtime-type dispatch, polymorphic case behavior, IAsyncEnumerable<T>, ConfigureHttpJsonOptions).
• Refactor existing JSON-body tests to use a new helper that consistently creates an empty JSON body request.

Show a summary per file

File	Description
src/Http/samples/MinimalSample/Program.cs	Adds sample endpoints/types demonstrating unions in body binding and as return values, including classifier examples.
src/Http/Http.Extensions/test/RequestDelegateGenerator/SharedTypes.Unions.cs	Introduces shared union types and classifier factories used by generator tests.
src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.Unions.Responses.cs	Adds union-focused response serialization tests for RDG/RDF.
src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.Unions.Body.cs	Adds union-focused JSON body binding tests for RDG/RDF.
src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.JsonBodyOrService.cs	Uses the new empty-body helper for consistency.
src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.JsonBody.cs	Uses the new empty-body helper for consistency.
src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTestBase.cs	Adds helpers for empty JSON body requests and custom content types.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 2