Update project and dependencies, enable nullable annotations

[tillig]

Brings the library up to current Autofac project standards ahead of the 7.0.0 release.

Changes

• Dependencies: Autofac 9.3.2, Autofac.Mef 8.0.1, System.ComponentModel.Composition 10.0.11, plus analyzer/test-tooling bumps and SDK 10.0.302.
• Nullable annotations enabled across source and tests. These are annotation-only changes and are binary compatible, since nullable metadata is erased at runtime and where TInterface : notnull (now required by Autofac's As<TService>()) is compiler-enforced only.
• Build metadata aligned with the other Autofac repos: full assembly/package metadata, snupkg symbols, resx-based strings, and the test project now multi-targets net10.0 and net8.0.
• README quick start rewritten. The previous one was copied from Autofac.Extras.AggregateService and documented aggregate services; since this branch adds PackageReadmeFile, that content would have shipped as the NuGet package page.

Notes

MetadataModule<TInterface, TMetadata>.ContainerBuilder is now ContainerBuilder?, so the public RegisterType/RegisterAttributedType methods guard it and throw InvalidOperationException when called before Load.

The README quick start centers on AttributedMetadataModule, since the filter attributes in this package (WithMetadataAttribute, WithKeyAttribute, WithAttributeFilter) are all [Obsolete] in favor of core Autofac's Autofac.Features.AttributeFilters equivalents. Snippets are adapted from the metadata docs and were verified against a scratch test before being committed.

Test fix

Does_not_throw_in_nested_lifetimeScope_builders was failing on the new Autofac. It called builder.RegisterType<int>() inside the child scope's configuration action - two latent bugs from the original 2019 commit:

1. int is a struct, and core Autofac's RegistrationBuilder.ForType<T>() now rejects non-activatable types, so this threw instead of silently creating a registration nobody resolved.
2. It registered into the outer, already-built builder instead of the action's ContainerBuilder, so the nested registration the test name promises never happened.

Now x.RegisterType<NestedLifetimeScopeRegistrationInstance>(). Confirmed the test still guards its original regression by temporarily removing the ContainsKey guard from AttributedMetadataModule, which reproduces the duplicate-key ArgumentException (nested scopes see parent registrations wrapped in ExternalComponentRegistration, which shares the parent's metadata dictionary by reference).

Verification

dotnet msbuild ./default.proj clean: 30/30 tests pass on net10.0 and net8.0, packages build, and the correct README is packed into the nupkg.

[codecov]

Codecov Report

❌ Patch coverage is 39.13043% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.18%. Comparing base (61cef14) to head (2203f30).

Files with missing lines	Patch %	Lines
...teMetadata/MetadataModule{TInterface,TMetadata}.cs	22.22%	7 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop      #20       +/-   ##
============================================
- Coverage    71.27%   60.18%   -11.10%     
============================================
  Files            7        7               
  Lines           94      108       +14     
  Branches        28       35        +7     
============================================
- Hits            67       65        -2     
- Misses          12       19        +7     
- Partials        15       24        +9     

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