feat: migrate Inspect API to FastAPI router (#4403)

# What does this PR do?

Migrate the Inspect API to the FastAPI router pattern.

Changes:
- Add inspect API to FastAPI router registry
- Add PUBLIC_ROUTE_KEY support for routes that don't require auth
- Update WebMethod creation to respect route's openapi_extra for
authentication requirements

Fixes: https://github.com/llamastack/llama-stack/issues/4346

<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->

## Test Plan

CI and various curls on /v1/inspect/routes, /v1/health, /v1/version

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

docs/static/experimental-llama-stack-spec.yaml

@@ -2980,6 +2980,7 @@ components:
       properties:
         status:
           $ref: '#/components/schemas/HealthStatus'
+          description: The health status of the service
       type: object
       required:
       - status
@@ -2990,14 +2991,17 @@ components:
         route:
           type: string
           title: Route
+          description: The API route path
         method:
           type: string
           title: Method
+          description: The HTTP method for the route
         provider_types:
           items:
             type: string
           type: array
           title: Provider Types
+          description: List of provider types implementing this route
       type: object
       required:
       - route
@@ -3012,6 +3016,7 @@ components:
             $ref: '#/components/schemas/RouteInfo'
           type: array
           title: Data
+          description: List of available API routes
       type: object
       required:
       - data
@@ -6610,6 +6615,7 @@ components:
         version:
           type: string
           title: Version
+          description: The version string of the service
       type: object
       required:
       - version