Promotes the previously hardcoded transport-layer settings to ModbusDriverOptions
so users can tune them through DriverConfig JSON without recompiling.
Three new option groups:
1. KeepAlive (ModbusKeepAliveOptions): Enabled / Time / Interval / RetryCount.
Defaults preserve the historical PR 53 wire output exactly (Enabled=true,
Time=30s, Interval=10s, RetryCount=3). Set Enabled=false for PLCs that
reject SO_KEEPALIVE.
2. IdleDisconnectTimeout (TimeSpan?): when set, the transport tracks last-PDU-
success and proactively closes + reconnects on the next request after the
threshold. Defends against silent NAT / firewall socket reaping. Default
null = disabled (no behaviour change).
3. Reconnect (ModbusReconnectOptions): InitialDelay / MaxDelay /
BackoffMultiplier for the post-drop reconnect loop. Defaults
(InitialDelay=0, MaxDelay=30s, Multiplier=2.0) preserve the historical
immediate-retry behaviour for the first attempt and add geometric backoff
only if the reconnect itself fails. Capped at 10 attempts before propagating.
ModbusTcpTransport ctor extended with optional keepAlive / idleDisconnect /
reconnect parameters; existing 4-arg call sites continue to compile. Factory
DTO gains parallel KeepAlive / IdleDisconnectMs / Reconnect fields with
default-aware binding.
5 new ModbusConnectionOptionsTests covering the default-preservation contract
(every default field matches pre-#139) and the JSON-binding round-trip for
each knob group. Existing 204 unit tests still green.
