feat: migrate Providers API to FastAPI router pattern (#4405)

# What does this PR do?

Convert Providers API from @webmethod decorators to FastAPI router
pattern.

Fixes: https://github.com/llamastack/llama-stack/issues/4350

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relevant issues if applicable. -->

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## Test Plan
CI

Signed-off-by: Sébastien Han <seb@redhat.com>

src/llama_stack/core/providers.py

@@ -10,7 +10,14 @@ from typing import Any
 from pydantic import BaseModel
 
 from llama_stack.log import get_logger
-from llama_stack_api import HealthResponse, HealthStatus, ListProvidersResponse, ProviderInfo, Providers
+from llama_stack_api import (
+    HealthResponse,
+    HealthStatus,
+    InspectProviderRequest,
+    ListProvidersResponse,
+    ProviderInfo,
+    Providers,
+)
 
 from .datatypes import StackConfig
 from .utils.config import redact_sensitive_fields
@@ -67,13 +74,13 @@ class ProviderImpl(Providers):
 
         return ListProvidersResponse(data=ret)
 
-    async def inspect_provider(self, provider_id: str) -> ProviderInfo:
+    async def inspect_provider(self, request: InspectProviderRequest) -> ProviderInfo:
         all_providers = await self.list_providers()
         for p in all_providers.data:
-            if p.provider_id == provider_id:
+            if p.provider_id == request.provider_id:
                 return p
 
-        raise ValueError(f"Provider {provider_id} not found")
+        raise ValueError(f"Provider {request.provider_id} not found")
 
     async def get_providers_health(self) -> dict[str, dict[str, HealthResponse]]:
         """Get health status for all providers.

src/llama_stack/core/server/fastapi_router_registry.py

@@ -17,7 +17,7 @@ from fastapi import APIRouter
 from fastapi.routing import APIRoute
 from starlette.routing import Route
 
-from llama_stack_api import batches, benchmarks, datasets
+from llama_stack_api import batches, benchmarks, datasets, providers
 
 # Router factories for APIs that have FastAPI routers
 # Add new APIs here as they are migrated to the router system
@@ -27,6 +27,7 @@ _ROUTER_FACTORIES: dict[str, Callable[[Any], APIRouter]] = {
     "batches": batches.fastapi_routes.create_router,
     "benchmarks": benchmarks.fastapi_routes.create_router,
     "datasets": datasets.fastapi_routes.create_router,
+    "providers": providers.fastapi_routes.create_router,
 }