debug: 422 handler 简化 — 只记录 errors 和 content_type，不读 body

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/main.py

@@ -572,14 +572,13 @@ from fastapi.exceptions import RequestValidationError
 async def validation_error_handler(request: Request, exc: RequestValidationError):
     import logging
     _val_logger = logging.getLogger("nexus.validation")
-    try:
-        body = await request.body()
-        _val_logger.warning(
-            "422 validation error on %s %s body=%s errors=%s",
-            request.method, request.url.path, body[:500], exc.errors(),
-        )
-    except Exception:
-        _val_logger.warning("422 validation error on %s %s errors=%s", request.method, request.url.path, exc.errors())
+    client_host = request.client.host if request.client else "unknown"
+    content_type = request.headers.get("content-type", "")
+    errors = exc.errors()
+    _val_logger.warning(
+        "422 %s %s from %s content_type=%s errors=%s",
+        request.method, request.url.path, client_host, content_type, errors,
+    )
     from fastapi.responses import JSONResponse
     return JSONResponse(status_code=422, content={"detail": exc.errors()})