refactor(runtime): delete the dead in-place ApplyDelta seam; config change = respawn only (#665)

Since #516 a DriverConfig change is stop + respawn, so DriverSpawnPlan.ToApplyDelta
was always empty and the in-place path had no producer. #663 and #664 were both
latent bugs in it. This deletes it:
- host: ApplyChildDelta, _pendingDelta, HandleApplyResult and HandleDeltaApplied
- plan: DriverSpawnPlan.ToApplyDelta
- actor: the ApplyDelta, ApplyResult and DeltaApplied messages, the Connected
  reinit handler and the mid-connect AdoptConfigDuringInit
- tests: the actor tests that drove the seam

IDriver.ReinitializeAsync and each driver's implementation and unit tests stay.
Comments and docs that cited AdoptConfigDuringInit as the reason a driver must
release prior state at init now cite the IDriver contract and overlapping
retries, which still apply.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs

@@ -21,11 +21,11 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.MTConnect;
 ///         Agent. Every connection is owned by <see cref="InitializeAsync"/>.
 ///     </para>
 ///     <para>
-///         <b>The <c>driverConfigJson</c> argument is honoured, not decorative.</b> The runtime hands
-///         a config change to a <i>live</i> instance — <c>DriverInstanceActor</c>'s <c>ApplyDelta</c>
-///         calls <see cref="ReinitializeAsync"/> with the new document and never rebuilds the object
-///         — so a driver that read only its constructor options would turn every MTConnect config
-///         edit into a silent no-op that still seals the deployment green. This class therefore
+///         <b>The <c>driverConfigJson</c> argument is honoured, not decorative.</b> <see cref="IDriver.InitializeAsync"/>
+///         and <see cref="ReinitializeAsync"/> both take the document, and the contract is that the driver
+///         serves it — a driver that read only its constructor options would turn an in-place config
+///         change into a silent no-op. (The runtime itself now applies a config change by respawning,
+///         #516/#665.) This class therefore
 ///         re-parses the document (<see cref="ParseOptions"/>) whenever it carries a real body,
 ///         matching S7/AbCip/TwinCAT/Galaxy rather than the Modbus/FOCAS/OpcUaClient drivers that
 ///         ignore the argument. A blank / <c>{}</c> / <c>[]</c> document means "no config supplied"