feat(mqtt): MqttDriverForm + MqttDeviceForm — the driver/device config forms P1 omitted

The P1 live gate had to insert MQTT driver config directly via SQL: DriverConfigModal and
DeviceModal both fell through to "No typed config form for driver type Mqtt" with no raw-JSON
fallback, so broker host/port/TLS/credentials were unauthorable from the AdminUI. Task 12 built
the *tag* editor; these are the driver + device surfaces.

MqttDriverForm authors the whole MqttDriverOptions connection surface (host/port/clientId,
TLS + CA pin, credentials, protocol version/clean-session/keep-alive, connect timeout, reconnect
backoffs, mode, maxPayloadBytes, and the Plain sub-object). The Sparkplug sub-object stays P2 —
a marked placeholder mirroring MqttTagConfigEditor's stub, with any existing sparkplug keys
preserved untouched.

The connection is authored on the DRIVER, not the device — unlike Modbus/S7/OpcUaClient and
contrary to what docs/drivers/Mqtt.md said. DriverDeviceConfigMerger merges a device's keys up
only when the driver has exactly ONE device, so a device-authored broker connection vanishes
silently the moment a second device is added. MqttDeviceForm is therefore informational (the
GalaxyDeviceForm shape) and round-trips DeviceConfig verbatim; it flags legacy connection keys
left on a device by a pre-form deployment, because those still win via merge-up.

Serialization goes through the ONE shared MqttJson.Options instance (Task 9's decision), never a
fifth per-form copy — pinned by an assertion that an ordinal-only reader CANNOT bind the emitted
blob, and by extending the fleet-wide DriverPageJsonConverterTests guard to resolve a form's
serializer from an explicit external-instance registry when it has no _jsonOpts field. Dropping
the converter from MqttJson.Options reddens 3 tests and leaves the symmetric round-trip green —
the Task 9 lesson, reproduced.

RawTags is stripped from the emitted blob (the deploy artifact owns it) and unknown top-level
keys survive a load->save. Validation mirrors the driver's own [Range] bounds inline, and
ToOptions() additionally clamps, so an ignored error still cannot persist a
connectTimeoutSeconds:0 driver-brick. A non-blank clientId raises the P1 live-gate warning
inline (fixed ids make redundant pair nodes evict each other while both report Healthy).

AdminUI 761/761 green (was 730), TWAE-clean; Driver.Mqtt 266/266 green.

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

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

@@ -57,14 +57,30 @@
                             case DriverTypeNames.Galaxy:
                                 <GalaxyDriverForm @bind-DriverConfigJson="_driverConfigJson" @bind-ResilienceConfig="_resilienceConfig" />
                                 break;
+                            case DriverTypeNames.Mqtt:
+                                <MqttDriverForm @bind-DriverConfigJson="_driverConfigJson" @bind-ResilienceConfig="_resilienceConfig" />
+                                break;
                             default:
                                 <div class="alert alert-warning">No typed config form for driver type <span class="mono">@_driverType</span>.</div>
                                 break;
                         }
 
-                        <p class="form-text mt-3 mb-0">
-                            The connection endpoint + Test-connect live on the driver's <strong>device</strong> — open the device modal to author host/port and verify connectivity.
-                        </p>
+                        @* Galaxy + MQTT hold ONE connection per driver instance, so they author it here and
+                           their device forms are informational. Every other driver splits the endpoint onto
+                           the device (the v3 endpoint→DeviceConfig split). *@
+                        @if (_driverType is DriverTypeNames.Galaxy or DriverTypeNames.Mqtt)
+                        {
+                            <p class="form-text mt-3 mb-0">
+                                This driver holds a single connection, authored above. Test-connect lives on its
+                                <strong>device</strong> — open the device modal to verify connectivity.
+                            </p>
+                        }
+                        else
+                        {
+                            <p class="form-text mt-3 mb-0">
+                                The connection endpoint + Test-connect live on the driver's <strong>device</strong> — open the device modal to author host/port and verify connectivity.
+                            </p>
+                        }
 
                         @if (_saveError is not null)
                         {