feat: Implement FastAPI router system

This commit introduces a new FastAPI router-based system for defining
API endpoints, enabling a migration path away from the legacy @webmethod
decorator system. The implementation includes router infrastructure,
migration of the Batches API as the first example, and updates
to server, OpenAPI generation, and inspection systems to
support both routing approaches.

The router infrastructure consists of a router registry system
that allows APIs to register FastAPI router factories, which
are then automatically discovered and included in the server
application. Standard error responses are centralized in
router_utils to ensure consistent OpenAPI specification
generation with proper $ref references to component responses.

The Batches API has been migrated to demonstrate the new
pattern. The protocol definition and models remain in
llama_stack_api/batches, maintaining clear separation between
API contracts and server implementation. The FastAPI router
implementation lives in
llama_stack/core/server/routers/batches, following the
established pattern where API contracts are defined in
llama_stack_api and server routing logic lives in
llama_stack/core/server.

The server now checks for registered routers before falling
back to the legacy webmethod-based route discovery, ensuring
backward compatibility during the migration period. The
OpenAPI generator has been updated to handle both router-based
and webmethod-based routes, correctly extracting metadata from
FastAPI route decorators and Pydantic Field descriptions. The
inspect endpoint now includes routes from both systems, with
proper filtering for deprecated routes and API levels.

Response descriptions are now explicitly defined in router decorators,
ensuring the generated OpenAPI specification matches the
previous format. Error responses use $ref references to
component responses (BadRequest400, TooManyRequests429, etc.)
as required by the specification. This is neat and will allow us to
remove a lot of boiler plate code from our generator once the
migration is done.

This implementation provides a foundation for incrementally migrating
other APIs to the router system while maintaining full backward
compatibility with existing webmethod-based APIs.

Closes: https://github.com/llamastack/llama-stack/issues/4188
Signed-off-by: Sébastien Han <seb@redhat.com>

client-sdks/stainless/openapi.yml

@@ -37,7 +37,7 @@ paths:
           description: Default Response
       tags:
       - Batches
-      summary: List Batches
+      summary: List all batches for the current user.
       description: List all batches for the current user.
       operationId: list_batches_v1_batches_get
       parameters:
@@ -76,9 +76,11 @@ paths:
         default:
           $ref: '#/components/responses/DefaultError'
           description: Default Response
+        '409':
+          description: 'Conflict: The idempotency key was previously used with different parameters.'
       tags:
       - Batches
-      summary: Create Batch
+      summary: Create a new batch for processing multiple API requests.
       description: Create a new batch for processing multiple API requests.
       operationId: create_batch_v1_batches_post
       requestBody:
@@ -97,20 +99,20 @@ paths:
               schema:
                 $ref: '#/components/schemas/Batch'
         '400':
-          description: Bad Request
           $ref: '#/components/responses/BadRequest400'
+          description: Bad Request
         '429':
-          description: Too Many Requests
           $ref: '#/components/responses/TooManyRequests429'
+          description: Too Many Requests
         '500':
-          description: Internal Server Error
           $ref: '#/components/responses/InternalServerError500'
+          description: Internal Server Error
         default:
-          description: Default Response
           $ref: '#/components/responses/DefaultError'
+          description: Default Response
       tags:
       - Batches
-      summary: Retrieve Batch
+      summary: Retrieve information about a specific batch.
       description: Retrieve information about a specific batch.
       operationId: retrieve_batch_v1_batches__batch_id__get
       parameters:
@@ -119,7 +121,7 @@ paths:
         required: true
         schema:
           type: string
-        description: 'Path parameter: batch_id'
+          title: Batch Id
   /v1/batches/{batch_id}/cancel:
     post:
       responses:
@@ -130,20 +132,20 @@ paths:
               schema:
                 $ref: '#/components/schemas/Batch'
         '400':
-          description: Bad Request
           $ref: '#/components/responses/BadRequest400'
+          description: Bad Request
         '429':
-          description: Too Many Requests
           $ref: '#/components/responses/TooManyRequests429'
+          description: Too Many Requests
         '500':
-          description: Internal Server Error
           $ref: '#/components/responses/InternalServerError500'
+          description: Internal Server Error
         default:
-          description: Default Response
           $ref: '#/components/responses/DefaultError'
+          description: Default Response
       tags:
       - Batches
-      summary: Cancel Batch
+      summary: Cancel a batch that is in progress.
       description: Cancel a batch that is in progress.
       operationId: cancel_batch_v1_batches__batch_id__cancel_post
       parameters:
@@ -152,7 +154,7 @@ paths:
         required: true
         schema:
           type: string
-        description: 'Path parameter: batch_id'
+          title: Batch Id
   /v1/chat/completions:
     get:
       responses:
@@ -3950,29 +3952,35 @@ components:
         input_file_id:
           type: string
           title: Input File Id
+          description: The ID of an uploaded file containing requests for the batch.
         endpoint:
           type: string
           title: Endpoint
+          description: The endpoint to be used for all requests in the batch.
         completion_window:
           type: string
           const: 24h
           title: Completion Window
+          description: The time window within which the batch should be processed.
         metadata:
           anyOf:
           - additionalProperties:
               type: string
             type: object
           - type: 'null'
+          description: Optional metadata for the batch.
         idempotency_key:
           anyOf:
           - type: string
           - type: 'null'
+          description: Optional idempotency key. When provided, enables idempotent behavior.
       type: object
       required:
       - input_file_id
       - endpoint
       - completion_window
       title: CreateBatchRequest
+      description: Request model for creating a batch.
     Batch:
       properties:
         id: