test: prove Polly resilience handler retries Google's HTTP 429

[maximn]

Summary

AddGoogleMaps already returns an IHttpClientBuilder, so .AddStandardResilienceHandler() from Microsoft.Extensions.Http.Resilience composes onto the Google Maps client without the core library taking any dependency on Polly. This PR proves that path with a regression test and documents it.

Related issue

n/a

Changes

• Add GoogleMapsApi.Extensions.DependencyInjection.Test/ResilienceTests.cs: a hermetic test proving the standard handler retries a 429 then succeeds (429 → retry → 200), plus a smoke test that it chains off every AddGoogleMaps overload.
• Reference Microsoft.Extensions.Http.Resilience 10.7.0 in the test project only (no new dependency on the core or DI library).
• Expand README.md and .agents/dependency-injection.md with the install step, chaining example, and a tuning note (429/408/5xx retried by default, Retry-After honored, DisableForUnsafeHttpMethods() for writes).

Test plan

• dotnet test on the DI test project: 9/9 pass on net8.0 and net10.0 (7 existing + 2 new); the 429 test asserts the fake handler was invoked twice, proving the retry fired.

Checklist

• dotnet format has been run.
• dotnet test passes locally (DI test project; no GOOGLE_API_KEY needed — tests are hermetic).
• Multi-framework build passes (netstandard2.0, net8.0, net10.0).
• XML documentation updated if public API surface changed. (No public API change.)

[github-actions]

🔬 TestGlance

✅ 251 passed across 1 job — 95.8% · ⏱️ 12.1s

Job	Result	Pass rate	Duration	Health
build	✅	251/262 · 95.8%	12.1s	—	Report

🟡 build — details

✅ 251 passed · ⏭️ 11 skipped
███████████████░ 95.8%
⏱️ 12.1s

vs master

Metric	master	PR	Delta
Pass rate	95.8%	95.8%	+0.0%
Duration	10.7s	12.1s	+13.0%

📄 HTML Report

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Updated 2026-06-15T19:36:57.980Z