feat: modularize fast_api.py to allow simpler construction of API Server

PiperOrigin-RevId: 783818634

Author: Poggecci

src/google/adk/cli/adk_web_server.py

@@ -31,7 +31,7 @@
 from ..sessions.in_memory_session_service import InMemorySessionService
 from ..sessions.session import Session
 from .utils import envs
-from .utils.agent_loader import AgentLoader
+from .utils.file_system_agent_loader import FileSystemAgentLoader
 
 
 class InputFile(BaseModel):
@@ -137,7 +137,7 @@ async def run_cli(
   session = await session_service.create_session(
       app_name=agent_folder_name, user_id=user_id
   )
-  root_agent = AgentLoader(agents_dir=agent_parent_dir).load_agent(
+  root_agent = FileSystemAgentLoader(agents_dir=agent_parent_dir).load_agent(
       agent_folder_name
   )
   envs.load_dotenv_for_agent(agent_folder_name, agent_parent_dir)

src/google/adk/cli/cli.py

@@ -24,7 +24,7 @@
 from typing import Optional
 import uuid
 
-from ..agents import Agent
+from ..agents.llm_agent import Agent
 from ..artifacts.base_artifact_service import BaseArtifactService
 from ..evaluation.constants import MISSING_EVAL_DEPENDENCIES_MESSAGE
 from ..evaluation.eval_case import EvalCase