s7: explain V1-initial firmware-band fallback to users

When a real V1-initial S7-1200 (FW < 4.5) sends Struct(314) for
ServerSessionVersion instead of a scalar, our session-setup echo
can't run, the PLC drops the connection, and the unified Client
falls back to legacy PUT/GET. The previous warning ("PLC did not
provide ServerSessionVersion") was generic enough that users
couldn't tell whether it was a bug, a network issue, or an
unsupported firmware band.

Replace with an actionable message: explains the firmware band,
points at issue #710, and tells users explicitly that db_read /
db_write still work but browse() will not.

Refs #710.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: gijzelaerr

s7/connection.py

@@ -205,7 +205,14 @@ def connect(
             if self._server_session_version is not None:
                 self._session_setup_ok = self._setup_session()
             else:
-                logger.warning("PLC did not provide ServerSessionVersion - session setup incomplete")
+                logger.warning(
+                    "PLC did not provide a scalar ServerSessionVersion attribute. "
+                    "This is the V1-initial S7-1200 firmware band (FW < 4.5 "
+                    "predating TLS) which sends a Struct(314) value and requires "
+                    "the proprietary SessionKey handshake — not yet implemented "
+                    "in python-snap7 (tracked in issue #710). Falling back to "
+                    "legacy PUT/GET: db_read/db_write will work, browse() will not."
+                )
                 self._session_setup_ok = False
 
             # Step 6: Version-specific post-setup