fix: adding missing import

doc: added TPM attestation related information

doc/api-reference.md

@@ -75,10 +75,30 @@ A dictionary containing the chat completion response with the following structur
         "prompt_tokens": int,
         "completion_tokens": int,
         "total_tokens": int
+    },
+    "_metadata": {
+        "payload_id": str,
+        "processed_at": int,          # Unix timestamp
+        "is_encrypted": bool,
+        "response_status": str,
+        "security_tier": str,         # "standard", "high", or "maximum"
+        "memory_protection": dict,    # server-side memory protection info
+        "cuda_device": dict,          # privacy-safe GPU info (hashed identifiers)
+        "tpm_attestation": {          # TPM 2.0 hardware attestation (see Security Guide)
+            "is_available": bool,
+            # Present only when is_available is True:
+            "pcr_banks": str,         # e.g. "sha256:0,7,10"
+            "pcr_values": dict,       # {bank: {pcr_index: hex_digest}}
+            "quote_b64": str,         # base64-encoded TPMS_ATTEST (signed by AIK)
+            "signature_b64": str,     # base64-encoded TPMT_SIGNATURE
+            "aik_pubkey_b64": str,    # base64-encoded TPM2B_PUBLIC (ephemeral AIK)
+        }
     }
 }
 ```
 
+The `_metadata` field is added by the client library and is not part of the OpenAI API response format. See the [Security Guide](security-guide.md) for how to interpret and verify `tpm_attestation`.
+
 #### acreate(model, messages, **kwargs)
 
 Async alias for create() method.