debug: 添加 422 验证错误日志 — 记录请求体和验证错误详情

诊断 Agent 心跳 422 问题，临时捕获请求体内容。

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

server/main.py

@@ -564,6 +564,25 @@ async def custom_http_exception_handler(request: Request, exc: StarletteHTTPExce
     from fastapi.responses import JSONResponse
     return JSONResponse(status_code=exc.status_code, content={"detail": exc.detail})
 
+
+# ── Debug: log 422 validation errors with request body ──
+from fastapi.exceptions import RequestValidationError
+
+@app.exception_handler(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())
+    from fastapi.responses import JSONResponse
+    return JSONResponse(status_code=422, content={"detail": exc.errors()})
+
 # ── Static files: serve web/app/ at /app/ (HTML pages + JS/CSS assets) ──
 # In production, Nginx serves these directly; this is for dev/standalone mode.
 WEB_APP_DIR = ROOT_DIR / "web" / "app"