Phase 6.1 Stream A — switch pipeline keys from Guid to string to match IDriver.DriverInstanceId

IDriver.DriverInstanceId is declared as string in Core.Abstractions; keeping
the pipeline key as Guid meant every call site would need .ToString() / Guid.Parse
at the boundary. Switching the Resilience types to string removes that friction
and lets OtOpcUaServer pass driver.DriverInstanceId directly to the builder in
the upcoming server-dispatch wiring PR.

- DriverResiliencePipelineBuilder.GetOrCreate + Invalidate + PipelineKey
- CapabilityInvoker.ctor + _driverInstanceId field

Tests: all 48 Core.Tests still pass. The Invalidate test's keepId / dropId now
use distinct "drv-keep" / "drv-drop" literals (previously both were distinct
Guid.NewGuid() values, which the sed-driven refactor had collapsed to the same
literal — caught pre-commit).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/ZB.MOM.WW.OtOpcUa.Core.Tests/Resilience/CapabilityInvokerTests.cs

@@ -11,7 +11,7 @@ public sealed class CapabilityInvokerTests
     private static CapabilityInvoker MakeInvoker(
         DriverResiliencePipelineBuilder builder,
         DriverResilienceOptions options) =>
-        new(builder, Guid.NewGuid(), () => options);
+        new(builder, "drv-test", () => options);
 
     [Fact]
     public async Task Read_ReturnsValue_FromCallSite()