feat: validazione expires_at ISO 8601 (422 con messaggio chiaro, niente fallback silenzioso)

- field_validator Pydantic su MemoryIn.expires_at
- verificato sul server: valida OK, invalida → 422 (istanza di test)

gateway/main.py

@@ -33,7 +33,7 @@ from typing import Any, Literal, Optional
 import httpx
 import uvicorn
 from fastapi import Depends, FastAPI, Header, HTTPException, Request
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_validator
 from qdrant_client import QdrantClient
 from qdrant_client.http import models as qm
 
@@ -135,6 +135,18 @@ class MemoryIn(BaseModel):
     supersedes_id: Optional[str] = None
     supersede_reason: Optional[str] = Field(default=None, max_length=512)
 
+    @field_validator("expires_at")
+    @classmethod
+    def _validate_expires_at(cls, v: Optional[str]) -> Optional[str]:
+        """Valida il formato ISO 8601: invalida → 422 (niente fallback silenzioso)."""
+        if v is None:
+            return v
+        try:
+            datetime.fromisoformat(v.replace("Z", "+00:00"))
+        except ValueError:
+            raise ValueError("expires_at deve essere una data ISO 8601 valida (es. 2026-09-01T00:00:00Z)")
+        return v
+
 
 class SearchIn(BaseModel):
     query: str = Field(min_length=1, max_length=512)