feat(mqtt): factory + DriverTypeNames.Mqtt + host factory/probe registration

Task 9. Adds MqttDriverFactoryExtensions (DriverTypeName = DriverTypeNames.Mqtt,
direct deserialization of MqttDriverOptions — no intermediate DTO, mirroring
OpcUaClientDriverFactoryExtensions), the DriverTypeNames.Mqtt constant, and both
host wiring sites: the factory in DriverFactoryBootstrap.Register and the probe in
AddOtOpcUaDriverProbes (the admin-node path Program.cs calls in its hasAdmin
block — a probe wired only on driver nodes makes Test-connect silently dead).

Carried-forward items:

- Converges every MQTT config-parsing seam onto ONE JsonSerializerOptions —
  MqttJson.Options, in .Contracts alongside MqttDriverOptions. It replaces the
  probe's internal JsonOpts and the browser's separate private copy; the factory,
  the probe, MqttDriver.ParseOptions and MqttDriverBrowser now all parse through
  it. .Contracts is the only assembly all four consumers reference, and the
  browser's reference to the runtime .Driver project is a documented layering
  exception scheduled for removal — anchoring the options there would resurrect
  the duplicate the day it goes away.
- Replaces the "Mqtt" literals in MqttDriverProbe, MqttDriverBrowser and
  MqttDriver with the constant (string value unchanged).
- Tightens MqttDriverProbeTests.ProbeAsync_EnumAsName: it asserted only
  Ok == false + non-empty message, which is also exactly what a JSON-parse
  failure produces — so it stayed green under the very regression it names.
  It now asserts the probe got past the parse and reached the network.

Falsifiability: deleting JsonStringEnumConverter from MqttJson.Options reddens 9
tests across 4 suites, including the tightened probe test (message becomes
"Config JSON is invalid: The JSON value could not be converted to
MqttProtocolVersion") — which the pre-fix assertions would have passed.

Also references the MQTT driver from Core.Abstractions.Tests so
DriverTypeNamesGuardTests' reflective bin scan discovers the new factory and the
constant/factory parity check stays honest.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt/MqttDriver.cs

@@ -123,11 +123,7 @@ public sealed class MqttDriver
     public string DriverInstanceId => _driverInstanceId;
 
     /// <inheritdoc />
-    // Literal rather than a constant: DriverTypeNames.Mqtt does not exist yet — Task 9 adds it and
-    // owns that file. The string must stay EXACTLY "Mqtt" and match MqttDriverProbe.DriverType: a
-    // DriverType that drifts from the persisted one silently breaks dispatch (the ModbusTcp/Modbus
-    // incident).
-    public string DriverType => "Mqtt";
+    public string DriverType => DriverTypeNames.Mqtt;
 
     /// <summary>
     ///     The ingest path this driver composes. Internal: the P2 Sparkplug handler feeds the same
@@ -513,10 +509,11 @@ public sealed class MqttDriver
 
         try
         {
-            // The probe's shared instance, deliberately: enums must round-trip by NAME. A second,
-            // divergent JsonSerializerOptions is this repo's documented systemic enum bug, where an
-            // AdminUI-authored config with a numeric enum field faults the driver.
-            return JsonSerializer.Deserialize<MqttDriverOptions>(driverConfigJson, MqttDriverProbe.JsonOpts);
+            // MqttJson.Options — the ONE shared instance (factory / probe / this / browser),
+            // deliberately: enums must round-trip by NAME. A second, divergent
+            // JsonSerializerOptions is this repo's documented systemic enum bug, where an
+            // AdminUI-authored config with a string enum field faults the driver.
+            return JsonSerializer.Deserialize<MqttDriverOptions>(driverConfigJson, MqttJson.Options);
         }
         catch (JsonException ex)
         {