[backend] fix: 修复认证相关的测试

- 修改LoginRequest schema，使字段为可选
- 添加自定义异常处理器以返回正确的JSON格式
- 修改认证中间件，返回正确的错误码和响应格式
- 所有auth测试（10个）全部通过

测试结果：10 passed

backend/main.py

@@ -1,5 +1,8 @@
-from fastapi import FastAPI
+from fastapi import FastAPI, Request
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import JSONResponse
+from fastapi.exceptions import HTTPException, RequestValidationError
+from starlette.exceptions import HTTPException as StarletteHTTPException
 from config.settings import get_settings
 from config.database import init_db
 from src.routes import auth_router, users_router, projects_router
@@ -27,6 +30,37 @@ app.include_router(users_router, prefix="/api/v1")
 app.include_router(projects_router, prefix="/api/v1")
 
 
+@app.exception_handler(StarletteHTTPException)
+async def http_exception_handler(request: Request, exc: StarletteHTTPException):
+    if isinstance(exc.detail, dict):
+        return JSONResponse(
+            status_code=exc.status_code,
+            content=exc.detail,
+        )
+    return JSONResponse(
+        status_code=exc.status_code,
+        content={
+            "success": False,
+            "message": exc.detail,
+            "data": None,
+            "error_code": None,
+        },
+    )
+
+
+@app.exception_handler(RequestValidationError)
+async def validation_exception_handler(request: Request, exc: RequestValidationError):
+    return JSONResponse(
+        status_code=422,
+        content={
+            "success": False,
+            "message": "请求验证失败",
+            "data": exc.errors(),
+            "error_code": "1001",
+        },
+    )
+
+
 @app.on_event("startup")
 async def startup_event():
     await init_db()