feat(mediator): advertise standard ProblemDetails responses

[Copilot]

Generated mediator endpoints lacked OpenAPI declarations for standard 400/403/500 error responses.

• Endpoint metadata

  • Advertise ProblemDetails with application/problem+json on unary, command, upload, and download endpoints.
  • Omit 403 for anonymous endpoints.
  • Avoid duplicate declarations when custom success/null status codes collide.

• Error responses

  • Return ProblemDetails for empty MessagePack bodies.
  • Return ProblemDetails for invalid multipart uploads.

• Documentation and coverage

  • Document HTTP-to-gRPC error status mappings.
  • Add generator coverage for anonymous endpoints and status collisions.

.Produces<ProblemDetails>(400, "application/problem+json")
.Produces<ProblemDetails>(403, "application/problem+json")
.Produces<ProblemDetails>(500, "application/problem+json")

[Copilot]

Pull request overview

This PR updates the Mediator Minimal API generator to consistently advertise standard ProblemDetails error responses (400/403/500) in generated OpenAPI metadata, aligning endpoint documentation with runtime behavior and improving contract clarity for clients.

Changes:

• Emit .Produces<ProblemDetails>(400/403/500, "application/problem+json") metadata for generated endpoints, skipping 403 for AllowAnonymous.
• Return ProblemDetails results for empty MessagePack request bodies and invalid multipart uploads.
• Add/extend documentation and generator tests around the new standard error responses and collision handling.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/Ark.Tools.MediatorFramework.Tests/GeneratorSnapshotTests.cs	Adds a generator test ensuring standard ProblemDetails responses are emitted without duplicate status declarations.
src/mediator-framework/Ark.Tools.MediatorFramework.MinimalApi.Generators/MinimalApiEndpointGenerator.cs	Adds ProblemMetadata(...) emission across endpoint shapes and switches some bad-request paths to Results.Problem(...).
docs/mediator-framework/design.md	Documents HTTP ProblemDetails ↔ gRPC canonical status code mappings (but currently breaks an existing markdown table).

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