fix(aspnetcore): resolve ResultEndpointFilter mapper per request

[kathelon]

Summary

• resolve IResultErrorMapper from HttpContext.RequestServices inside ResultEndpointFilter
• keep the existing fallback behavior when no mapper is registered, including null RequestServices
• add tests for request-scoped mapper resolution on both generic and non-generic Result failures
• refresh the ASP.NET Core README snippet and sync the root test badge count

Validation

• dotnet test CSharpEssentials.Tests/CSharpEssentials.Tests.csproj --filter ResultEndpointFilterTests
• dotnet test
• dotnet build --no-restore

Closes #6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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