feat: add a2a routes to existing app (#188)

- Adds a method to add a2a routes to an existing app (Starlette or
FastAPI)
- There are some use-cases where an app already exists and we simply
need to add a2a routes to that existing app
- Refactors `build` method to use the above method

---------

Co-authored-by: MadaraUchiha-314 <rohithr31@gmail.com>

Author: MadaraUchiha-314

src/a2a/server/apps/jsonrpc/fastapi_app.py

@@ -49,25 +49,21 @@ def __init__(
             context_builder=context_builder,
         )
 
-    def build(
+    def add_routes_to_app(
         self,
+        app: FastAPI,
         agent_card_url: str = '/.well-known/agent.json',
         rpc_url: str = '/',
         extended_agent_card_url: str = '/agent/authenticatedExtendedCard',
-        **kwargs: Any,
-    ) -> FastAPI:
-        """Builds and returns the FastAPI application instance.
+    ) -> None:
+        """Adds the routes to the FastAPI application.
 
         Args:
+            app: The FastAPI application to add the routes to.
             agent_card_url: The URL for the agent card endpoint.
             rpc_url: The URL for the A2A JSON-RPC endpoint.
             extended_agent_card_url: The URL for the authenticated extended agent card endpoint.
-            **kwargs: Additional keyword arguments to pass to the FastAPI constructor.
-
-        Returns:
-            A configured FastAPI application instance.
         """
-        app = FastAPI(**kwargs)
 
         @app.post(rpc_url)
         async def handle_a2a_request(request: Request) -> Response:
@@ -85,4 +81,28 @@ async def get_extended_agent_card(request: Request) -> Response:
                     request
                 )
 
+    def build(
+        self,
+        agent_card_url: str = '/.well-known/agent.json',
+        rpc_url: str = '/',
+        extended_agent_card_url: str = '/agent/authenticatedExtendedCard',
+        **kwargs: Any,
+    ) -> FastAPI:
+        """Builds and returns the FastAPI application instance.
+
+        Args:
+            agent_card_url: The URL for the agent card endpoint.
+            rpc_url: The URL for the A2A JSON-RPC endpoint.
+            extended_agent_card_url: The URL for the authenticated extended agent card endpoint.
+            **kwargs: Additional keyword arguments to pass to the FastAPI constructor.
+
+        Returns:
+            A configured FastAPI application instance.
+        """
+        app = FastAPI(**kwargs)
+
+        self.add_routes_to_app(
+            app, agent_card_url, rpc_url, extended_agent_card_url
+        )
+
         return app

src/a2a/server/apps/jsonrpc/starlette_app.py

@@ -92,6 +92,28 @@ def routes(
             )
         return app_routes
 
+    def add_routes_to_app(
+        self,
+        app: Starlette,
+        agent_card_url: str = '/.well-known/agent.json',
+        rpc_url: str = '/',
+        extended_agent_card_url: str = '/agent/authenticatedExtendedCard',
+    ) -> None:
+        """Adds the routes to the Starlette application.
+
+        Args:
+            app: The Starlette application to add the routes to.
+            agent_card_url: The URL path for the agent card endpoint.
+            rpc_url: The URL path for the A2A JSON-RPC endpoint (POST requests).
+            extended_agent_card_url: The URL for the authenticated extended agent card endpoint.
+        """
+        routes = self.routes(
+            agent_card_url=agent_card_url,
+            rpc_url=rpc_url,
+            extended_agent_card_url=extended_agent_card_url,
+        )
+        app.routes.extend(routes)
+
     def build(
         self,
         agent_card_url: str = '/.well-known/agent.json',
@@ -110,14 +132,10 @@ def build(
         Returns:
             A configured Starlette application instance.
         """
-        app_routes = self.routes(
-            agent_card_url=agent_card_url,
-            rpc_url=rpc_url,
-            extended_agent_card_url=extended_agent_card_url,
+        app = Starlette(**kwargs)
+
+        self.add_routes_to_app(
+            app, agent_card_url, rpc_url, extended_agent_card_url
         )
-        if 'routes' in kwargs:
-            kwargs['routes'].extend(app_routes)
-        else:
-            kwargs['routes'] = app_routes
 
-        return Starlette(**kwargs)
+        return app