Make S7CommPlus session setup and SessionKey activation atomic

[gijzelaerr]

Problem

The sync client installs self._session_key and enables integrity counters before the PLC accepts SetupSession. Both sync and async clients can then set connected = True even when session setup returns false.

This produces an invalid public state: transport TCP/COTP may be connected, but the S7CommPlus session is unusable or locally authenticated with a key the PLC rejected.

Scope

• Keep generated key material pending until SetupSession succeeds.
• Commit the key and integrity state only after validating the matching successful response.
• Treat mandatory session-setup failure as a connection failure.
• Ensure cleanup clears pending and active authentication state.
• Define internal transport-connected versus protocol-session-ready states without changing unrelated public APIs.

Acceptance criteria

• connected is never true after mandatory SetupSession rejection or malformed response.
• Rejected authentication does not leave _session_key or integrity tracking active.
• Successful legacy and TLS setup still transition to connected exactly once.
• Disconnect is safe from every intermediate state.
• Tests cover success, PLC error, malformed response and socket failure during setup.

Out of scope

• HMAC verification itself
• Key renewal
• Async legacy-auth parity