Add lightweight typed configuration precedence sample

[balajibashyam]

Summary

Adds a minimal .NET 10 companion sample for the .NET 8 Configuration & Secrets Management: Typed Options, User Secrets & Feature Flags tutorial.

What's included

• New sample: dotnet-8-essentials/configuration-secrets-environments/ — 12 files

  • Layered configuration precedence (appsettings.json → appsettings.Development.json → User Secrets → CFGPLAY_ environment variables → command-line arguments)
  • Strongly typed AppOptions with DataAnnotations validation
  • Custom IValidateOptions<AppOptions> validator (HTTPS enforcement, placeholder API-key rejection)
  • ValidateOnStart for early startup failure on bad configuration
  • Lightweight FeatureFlagOptions with IOptionsSnapshot
  • Safe Development-only diagnostics endpoint with explicit allowlist and [REDACTED] API key
  • 8 integration tests covering provider precedence, validation, secret non-disclosure, and startup failure
  • Zero direct NuGet package references in the application project

• Root README: New sample row in the table + updated repository structure tree

• GitHub Actions: New test-config-precedence job in the shared workflow; path filters updated

Deliberately omitted (stays in the full tutorial)

Azure Key Vault, named options, IOptionsMonitor callbacks, live file reload, raw provider debug trees, external services, database configuration, SMTP configuration

Versions

• .NET SDK: 10.0.302
• Microsoft.AspNetCore.Mvc.Testing: 10.0.10
• Microsoft.NET.Test.Sdk: 18.8.1
• xunit.v3: 3.2.2
• xunit.runner.visualstudio: 3.1.5
• Application NuGet dependencies: none

Validation

• dotnet restore — passed
• dotnet build --configuration Release — passed (0 errors, 0 warnings)
• dotnet test --configuration Release — passed (8/8)
• Runtime precedence: timeout 40 (CLI wins over CFGPLAY_ 30, Dev 20, base 10)
• Runtime diagnostics: [REDACTED] confirmed, no CI key leaked
• Application package count: 0
• Security scan — clean
• YAML validation — passed