feat(health): core review fixes (async writer, gRPC cancellation, validation, configurable retry-after)

components/health/shared-contract/ZB.MOM.WW.Health.md

@@ -73,7 +73,7 @@ public static class ZbHealthEndpointExtensions
         Action<ZbHealthEndpointOptions> configure);
 }
 
-/// Canonical JSON response writer. Shape: { status, totalDurationMs, entries: { name: { status, description, duration } } }.
+/// Canonical JSON response writer. Shape: { status, totalDurationMs, entries: { name: { status, description, durationMs } } }.
 public static class ZbHealthWriter
 {
     public static Task WriteJsonAsync(HttpContext context, HealthReport report);

components/health/spec/SPEC.md

@@ -121,12 +121,12 @@ All health endpoints share one canonical JSON serializer. The shape is lifted fr
     "database": {
       "status": "Healthy",
       "description": "SQL Server reachable",
-      "duration": "00:00:00.0120000"
+      "durationMs": 12
     },
     "akka-cluster": {
       "status": "Healthy",
       "description": "Member status: Up",
-      "duration": "00:00:00.0001000"
+      "durationMs": 0.1
     }
   }
 }
@@ -141,7 +141,7 @@ All health endpoints share one canonical JSON serializer. The shape is lifted fr
 | `entries` | object | Keyed by check registration name |
 | `entries.<name>.status` | string | Per-check status |
 | `entries.<name>.description` | string? | Human-readable detail (may be null) |
-| `entries.<name>.duration` | string | TimeSpan `ToString()` — per-check elapsed time |
+| `entries.<name>.durationMs` | number | Per-check elapsed time, milliseconds |
 
 The writer is exposed as a static `Task WriteJsonAsync(HttpContext, HealthReport)` so consumers can
 plug it into `MapHealthChecks` options and also call it from custom endpoints.