test: Unit tests for OpenAPI metadata extensions (RouteHandlerBuilderExtensions, XmlDocumentationReader, global)

[samtrion]

User Story

As a developer maintaining Pulse.AspNetCore, I want unit tests for OpenAPI metadata extensions, so that I can detect regressions.

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Requirements

• Tests.Unit/AspNetCore/RouteHandlerBuilderExtensionsTests.cs: all five extension methods.
• Tests.Unit/AspNetCore/XmlDocumentationReaderTests.cs: XML present, XML absent, caching.
• Tests.Unit/AspNetCore/OpenApiMediatorBuilderExtensionsTests.cs: flag set correctly, auto-applied in Map* methods.

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Acceptance Criteria

• All tests pass.

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Dependencies

• Requires feat: OpenAPI metadata extension methods for MapCommand, MapQuery, MapStreamQuery