feat(api): nest chat endpoints under projects and refactor dependencies

- Change chat router prefix from `/chat` to `/projects/{project_id}/chat`
- Add `dep_project` dependency to inject Project instance into endpoints
- Remove manual project validation in `stream_chat` and use dependency instead
- Add placeholder `list_sessions` endpoint for future session listing
- Update `get_history` endpoint to use project dependency and adjust path
- Improve code organization and error handling for project retrieval

gns3server/api/routes/controller/__init__.py

@@ -156,6 +156,6 @@ router.include_router(
 
 router.include_router(
     chat.router,
-    prefix="/chat",
+    prefix="/projects/{project_id}/chat",
     tags=["Chat"]
 )