fix(ablegacy): coalesceReads absent means default; the form never pins it

Review r0 M9. The AdminUI form wrote coalesceReads:true on every save, pinning
today's default into every edited driver. AbLegacyDriverOptions.CoalesceReads
is now bool? (null = default ON, never serialised, same encoding as
Transport); the form omits the key when the switch is on and writes only an
explicit false. Persisted coalesceReads:true keeps loading as on.

Refs #738
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Drivers/Forms/AbLegacyDriverForm.razor

@@ -208,7 +208,7 @@
             _loadedProbeTimeout = o.Probe.Timeout,
             TimeoutSeconds = (int)o.Timeout.TotalSeconds,
             Transport = o.Transport,
-            CoalesceReads = o.CoalesceReads,
+            CoalesceReads = o.CoalesceReads ?? true,
             ProbeEnabled = o.Probe.Enabled,
             ProbeIntervalSeconds = (int)o.Probe.Interval.TotalSeconds,
             ProbeTimeoutSeconds = (int)o.Probe.Timeout.TotalSeconds,
@@ -229,7 +229,8 @@
             },
             Timeout = DriverConfigMs.Save(TimeoutSeconds, _loadedTimeout),
             Transport = Transport,
-            CoalesceReads = CoalesceReads,
+            // On = the default: OMIT the key (never pin a concrete default); off is the only value written.
+            CoalesceReads = CoalesceReads ? null : false,
             ProbeTimeoutSeconds = AdminProbeTimeoutSeconds,
         };
     }