OpenAPI Source Generation MVC Customization

[feO2x]

Closes #47

Implemented as stated in the corresponding issue.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Light.PortableResults	96%	94%	2301	✔
Light.PortableResults.AspNetCore.MinimalApis	93%	80%	24	✔
Light.PortableResults.AspNetCore.Mvc	93%	80%	24	✔
Light.PortableResults.AspNetCore.OpenApi	93%	74%	518	✔
Light.PortableResults.AspNetCore.Shared	100%	100%	26	✔
Light.PortableResults.Validation	96%	89%	1757	✔
Light.PortableResults.Validation.OpenApi	98%	90%	105	✔
Light.PortableResults.Validation.OpenApi.SourceGeneration	88%	83%	755	✔
Summary	95% (9356 / 9872)	89% (3950 / 4446)	5510	✔

Minimum allowed line rate is 60%

[Copilot]

Pull request overview

Adds an additive, attribute-friendly MVC customization path for generated validation OpenAPI metadata by introducing a second, two-contract MVC attribute that mirrors Minimal API ordering (generated validator contract first, endpoint-local customization second).

Changes:

• Added ProducesPortableValidationProblemForAttribute<TValidator, TEndpointContract> for MVC to apply generated metadata and then append endpoint-local builder customizations.
• Added unit + integration tests to verify metadata arrays append (rather than replace) and to assert MVC behavior matches an equivalent Minimal API configure callback.
• Updated README and added an implementation plan doc for issue #47.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/Light.PortableResults.Validation.OpenApi.Tests/ValidationOpenApiAttributeTests.cs	Adds a unit test proving endpoint-local contract metadata is appended after the generated contract.
tests/Light.PortableResults.Validation.OpenApi.Tests/GeneratedValidationOpenApiIntegrationTests.cs	Adds an integration test comparing MVC two-contract attribute output to an equivalent Minimal API customization.
src/Light.PortableResults.Validation.OpenApi/ProducesPortableValidationProblemForAttribute.cs	Introduces the new two-contract MVC attribute and applies contracts in the intended order using a shared builder.
README.md	Documents the new MVC customization pattern alongside the existing Minimal API callback customization.
ai-plans/0047-openapi-source-generation-mvc-customization.md	Adds the plan/acceptance-criteria document corresponding to issue #47.