Added server detection

Implemented server detection at the request of NTEWIZ. If not detected, it will ask after collecting pull data.

src/nte_history_exporter/decoder/server_region.py

@@ -0,0 +1,55 @@
+from __future__ import annotations
+
+import ipaddress
+import struct
+
+
+# Production entries from clientRes/OB-NTE-ServerList/serverlist_hhOS.json.
+# Test and certification servers are intentionally excluded.
+SERVER_REGIONS = {
+    "23001": {"account_region": "AS", "name": "Asia"},
+    "23002": {"account_region": "NA_SA", "name": "America"},
+    "23003": {"account_region": "EU", "name": "Europe"},
+    "23004": {"account_region": "SE", "name": "SEA"},
+}
+
+
+def account_region_for_server(server_id: str | None) -> str | None:
+    if not server_id:
+        return None
+    server = SERVER_REGIONS.get(str(server_id).strip())
+    return server["account_region"] if server else None
+
+
+def extract_server_id(payload: bytes) -> str | None:
+    """Extract DistrictId from the initial server-selection TCP response.
+
+    This response is a length-prefixed FlatBuffers message. DistrictId is at
+    byte 96 in the current schema. Validate the frame and its following server
+    address before accepting the value so an arbitrary integer at that offset
+    is not mistaken for a server ID.
+    """
+    if len(payload) < 168:
+        return None
+
+    frame_size = struct.unpack_from("<I", payload, 0)[0] + 4
+    if frame_size > len(payload) or frame_size < 168:
+        return None
+    if struct.unpack_from("<I", payload, 4)[0] != 20:
+        return None
+
+    server_id = struct.unpack_from("<I", payload, 96)[0]
+    if not 20_000 <= server_id <= 99_999:
+        return None
+
+    address_length = struct.unpack_from("<I", payload, 132)[0]
+    address_end = 136 + address_length
+    if not 1 <= address_length <= 255 or address_end > frame_size:
+        return None
+    try:
+        address = payload[136:address_end].decode("ascii")
+        ipaddress.ip_address(address)
+    except (UnicodeDecodeError, ValueError):
+        return None
+
+    return str(server_id)