feat: introduce /admin API for stack administration and operations (#4401)

# What does this PR do?

- Add new /admin API (v1alpha) for administrative operations including
provider management, health checks, version info, and route listing
- Implement using FastAPI routers following batches pattern with proper
request/response models
- Endpoints: /admin/providers, /admin/providers/{id},
/admin/inspect/routes, /admin/health, /admin/version
- Create admin module structure: models.py, api.py, fastapi_routes.py,
init.py
- Add AdminImpl in llama_stack/core combining provider and inspect
functionality
- Deprecate standalone /providers and /inspect APIs (remain functional
for backward compatibility)
- Consolidate duplicate types: ProviderInfo, HealthInfo, RouteInfo, etc.
now defined once in admin.models

## Test Plan

new admin integration suite, uses generated stainless SDK, and records
new tests on this PR.

Signed-off-by: Charlie Doern <cdoern@redhat.com>

client-sdks/stainless/config.yml

@@ -163,6 +163,12 @@ resources:
       sampling_params: SamplingParams
       scoring_result: ScoringResult
       system_message: SystemMessage
+      health_info: HealthInfo
+      provider_info: ProviderInfo
+      list_providers_response: ListProvidersResponse
+      route_info: RouteInfo
+      list_routes_response: ListRoutesResponse
+      version_info: VersionInfo
   toolgroups:
     models:
       tool_group: ToolGroup
@@ -261,11 +267,6 @@ resources:
             type: http
             endpoint: delete /v1/conversations/{conversation_id}/items/{item_id}
   inspect:
-    models:
-      healthInfo: HealthInfo
-      providerInfo: ProviderInfo
-      routeInfo: RouteInfo
-      versionInfo: VersionInfo
     methods:
       health: get /v1/health
       version: get /v1/version
@@ -357,16 +358,12 @@ resources:
             paginated: false
             endpoint: get /v1/models
   providers:
-    models:
-      list_providers_response: ListProvidersResponse
     methods:
       list:
         paginated: false
         endpoint: get /v1/providers
       retrieve: get /v1/providers/{provider_id}
   routes:
-    models:
-      list_routes_response: ListRoutesResponse
     methods:
       list:
         paginated: false
@@ -474,6 +471,13 @@ resources:
               cancel: delete /v1alpha/eval/benchmarks/{benchmark_id}/jobs/{job_id}
               status: get /v1alpha/eval/benchmarks/{benchmark_id}/jobs/{job_id}
               retrieve: get /v1alpha/eval/benchmarks/{benchmark_id}/jobs/{job_id}/result
+      admin:
+        methods:
+          list_providers: get /v1alpha/admin/providers
+          inspect_provider: get /v1alpha/admin/providers/{provider_id}
+          list_routes: get /v1alpha/admin/inspect/routes
+          health: get /v1alpha/admin/health
+          version: get /v1alpha/admin/version
   beta:
     subresources:
       datasets: