Add support for keyed DI services (Issue #165)

[avgalex]

Summary

• Validators registered via AddKeyedScoped, AddKeyedTransient, AddKeyedSingleton are now discovered automatically
• Registration order independent — works regardless of whether AddFluentValidationRulesToSwagger() is called before or after keyed validators are registered
• Graceful fallback when keyed services are not used or DI container doesn't support IKeyedServiceProvider
• Deduplication: same validator registered as both keyed and non-keyed is returned only once
• Version bumped to 7.0.4-beta.2

How it works

1. AddFluentValidationRulesToSwagger() registers IServiceCollection as a singleton snapshot
2. At resolution time, GetValidators() first resolves non-keyed validators (existing behavior)
3. Then scans the IServiceCollection snapshot for keyed IValidator<T> descriptors
4. Resolves each keyed validator via IKeyedServiceProvider.GetKeyedService()
5. Deduplicates by reference to avoid returning the same validator twice

Files changed

• ValidatorRegistryExtensions.cs — core resolution logic with new GetKeyedValidators helper
• ServiceCollectionExtensions.cs (Swashbuckle + NSwag) — register IServiceCollection singleton
• KeyedServicesTests.cs — 5 tests covering keyed-only, non-keyed regression, mixed, and full integration

Test plan

• All existing tests pass on net8.0, net9.0, net10.0
• 5 new keyed services tests pass on all frameworks
• Manual testing with sample application using keyed validators

Closes #165

🤖 Generated with Claude Code

[avgalex]

@claude review this PR

[claude]

Claude Code is working…

I'll analyze this and get back to you.

View job run

[avgalex]

Reopening from origin branch to fix CI fetch issue with fork PRs