Fix interface interception with open generic assembly scanning (#27)

[tillig]

Fixes #27.

Problem

EnableInterfaceInterceptors failed when used with open generic assembly scanning:

builder
  .RegisterAssemblyTypes(typeof(Program).Assembly)
  .AsClosedTypesOf(typeof(ICommandHandler<,>))
  .EnableInterfaceInterceptors()
  .InterceptedBy(typeof(LoggingInterceptor));

Resolving the closed interface threw an InvalidOperationException ("interface proxying only supports interface services").

The validation in EnsureInterfaceInterceptionApplies inspected every service exposed by the registration and threw if any was a non-interface. Because AsClosedTypesOf scanning also registers the concrete type as a service, the check tripped on the concrete type even when the interface was the thing being resolved.

Fix

Validate the service actually being resolved (ResolveRequestContext.Service) rather than every service on the registration. This is the approach discussed on the issue years ago; it's now possible because core Autofac exposes ctx.Service in the resolve pipeline. It's also strictly less heuristic than the previous logic.

The non-interface guard is preserved: directly resolving a concrete (non-interface) service registered with interface interception still throws.

Tests

New fixture OpenGenericInterfaceInterceptionFixture with three cases:

• InterceptsClosedTypesOfScannedOpenGenericInterface — the exact repro from the issue (failed before, passes now).
• InterceptsInterfaceServiceWhenRegistrationAlsoExposesConcreteType — AsSelf() + As<interface>(), resolved by interface, proxies correctly.
• ThrowsWhenResolvingConcreteServiceWithInterfaceInterception — confirms the non-interface guard still fires for direct concrete resolution.

The existing DetectsNonInterfaceServices guard test continues to pass. Full suite green on net8.0 and net10.0; clean Release build (warnings-as-errors + analyzers).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.34%. Comparing base (8f1268b) to head (12742a9).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #63      +/-   ##
===========================================
- Coverage    89.51%   89.34%   -0.18%     
===========================================
  Files            2        2              
  Lines          124      122       -2     
  Branches        22       21       -1     
===========================================
- Hits           111      109       -2     
  Misses           6        6              
  Partials         7        7              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.