Fix post-connect stability for Grizzl-E and similar OCPP 1.6 chargers

[lortiejp]

Summary

This PR fixes a post-connect instability seen with Grizzl-E chargers using OCPP 1.6.

In practice, the charger could connect, start boot/setup, then fall into reconnect/setup loops during the post-connect phase.

Changes

• serialize post_connect execution to avoid duplicate concurrent setup runs
• ignore duplicate post-connect triggers from boot notification / monitor backstop
• cancel pending post-connect work when the connection stops
• make GetConfiguration(HeartbeatInterval) timeout non-fatal
• avoid the extra MeterValuesSampledData query/configuration round-trip that can destabilize Grizzl-E chargers
• persist negotiated measurands and disable autoconfig after a successful negotiation
• add targeted tests for duplicate scheduling, heartbeat timeout handling, and measurand autodetect flow

Files

• custom_components/ocpp/chargepoint.py
• custom_components/ocpp/ocppv16.py
• tests/test_post_connect.py

Validation

Validated against a real Grizzl-E charger setup showing:

• stable OCPP connection
• successful StartTransaction / StopTransaction
• MeterValues received
• Heartbeat received

Local py_compile passed.

I was not able to complete full local pytest execution on macOS because of an old native dependency (lru-dict) failing to build there, so CI should be the source of truth for the full test run.