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_api/providers/models.py

@@ -0,0 +1,43 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""Pydantic models for Providers API requests and responses.
+
+This module defines the request and response models for the Providers API
+using Pydantic with Field descriptions for OpenAPI schema generation.
+"""
+
+from typing import Any
+
+from pydantic import BaseModel, Field
+
+from llama_stack_api.datatypes import HealthResponse
+from llama_stack_api.schema_utils import json_schema_type
+
+
+@json_schema_type
+class ProviderInfo(BaseModel):
+    """Information about a registered provider including its configuration and health status."""
+
+    api: str = Field(..., description="The API name this provider implements")
+    provider_id: str = Field(..., description="Unique identifier for the provider")
+    provider_type: str = Field(..., description="The type of provider implementation")
+    config: dict[str, Any] = Field(..., description="Configuration parameters for the provider")
+    health: HealthResponse = Field(..., description="Current health status of the provider")
+
+
+@json_schema_type
+class ListProvidersResponse(BaseModel):
+    """Response containing a list of all available providers."""
+
+    data: list[ProviderInfo] = Field(..., description="List of provider information objects")
+
+
+@json_schema_type
+class InspectProviderRequest(BaseModel):
+    """Request model for inspecting a provider by ID."""
+
+    provider_id: str = Field(..., description="The ID of the provider to inspect.")