Add lightweight Polly fallback sample

[balajibashyam]

Summary

Adds a minimal .NET 10 companion sample for the Polly Resilience (Polly v8): Timeouts, Retries, Circuits, Bulkheads, Hedging tutorial.

What's included

• New sample: cloud-native/polly-resilience/ — 13 files

  • Generic Polly v8 ResiliencePipeline<CatalogSnapshot> with fallback, timeout, and concurrency limiter
  • Explicit stale-cache fallback with marked degraded responses
  • Deterministic dependency simulation modes (live, failure, slow, hold)
  • Strategy event counters (fallback, timeout, rejection)
  • 6 integration tests covering fresh data, dependency failure, timeout, concurrency rejection, and validation

• Root README: New sample row in the table + updated cloud-native tree structure

• GitHub Actions: New test-polly-fallback-bulkhead job in the shared workflow; path filters updated for push and pull_request

Deliberately omitted (stays in the full tutorial)

• HTTP retries, circuit breakers, hedging
• External HTTP calls, databases, distributed caching
• OpenTelemetry, chaos testing, load testing
• Docker, Kubernetes
• Production tuning and capacity recommendations

Versions

• .NET SDK: 10.0.302
• Polly: 8.7.0
• Polly.RateLimiting: 8.7.0
• Microsoft.AspNetCore.Mvc.Testing: 10.0.10
• xunit.v3: 3.2.2

Validation

• dotnet restore — passed
• dotnet build --configuration Release — passed (0 errors, 0 warnings)
• dotnet test --configuration Release — passed (6/6)
• Runtime curl verification — live, failure fallback, timeout fallback all correct
• Security scan — clean