Preserve module metadata for custom route groups

[PANiXiDA]

What changed

• attach registered HttpModule metadata in the custom RouteGroupBuilder overload;
• cover custom root route groups with a regression unit test;
• document custom root route mapping for endpoints such as /connect/*.

Why

Module OpenAPI documents filter endpoints by HttpModule metadata. Custom route groups did not receive that metadata, so OAuth endpoints mapped under /connect disappeared from the Identity document.

Verification

• dotnet restore PANiXiDA.Core.Presentation.Http.slnx
• dotnet format PANiXiDA.Core.Presentation.Http.slnx --verify-no-changes --no-restore
• dotnet build PANiXiDA.Core.Presentation.Http.slnx --configuration Release --no-restore
• CI run: format, unit tests, report aggregation, and the 100% line coverage threshold passed

Compatibility

• migrations: none;
• public API signatures: unchanged;
• configuration changes: none;
• breaking changes: none.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Validate the service provider before dereferencing it

When serviceProvider is null, this newly added lookup throws NullReferenceException; for a group with no endpoint implementations, such as OtherEndpointGroup, the previous implementation never touched the provider and returned normally, while groups with endpoints delegated validation to ActivatorUtilities. Add an explicit ArgumentNullException.ThrowIfNull(serviceProvider) so this public argument fails deterministically rather than silently changing exception behavior.

AGENTS.md reference: AGENTS.md:L37-L37

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