refactor(drivers): delete GetMemoryFootprint / FlushOptionalCachesAsync (#525)

Both `IDriver` members had no production call site. Their only consumer was
`MemoryTracking`, deleted with the driver-tier recycle machinery in #522, and
`GetMemoryFootprint`'s doc-comment claimed "polled every 30s by Core" — which
had not been true since v3, even before #522 removed the poller.

Owner decision on #525 was (a) delete rather than (b) build the process-wide
watchdog that would consume them. The driver-side implementations are in git
history and were mostly reusable; what was always missing is the trigger, and a
member nobody calls is how the misleading doc-comment survived this long.

Removed from `IDriver`, all 12 driver implementations, and every test stub —
plus the support code that existed only to feed them, which is the part a
member-only deletion would have left behind as dead weight:

- MTConnect's three per-entry byte-weight constants, its cached
  `ProbeModelDataItemCount`, and `CountDataItems` — a full device-tree walk run
  on every cache fill to populate a number with no remaining reader.
- OpcUaClient's `_discoveredNodeCount` field and the discovery-pass write to it.
- MQTT's `ApproxBytesPerAuthoredTag`.

Two behavioural notes, neither a change in what the software does:

- `AbCipTemplateCache` now has no production reset path. Its only caller was
  AB CIP's flush, which itself had no caller — so nothing changes except how
  visible the fact is. Recorded on the type, along with the correction that its
  comment's claim about a device `ReinitializeAsync` also flushing it was never
  true. A UDT redefined on a live controller keeps its stale shape until the
  driver instance is rebuilt.
- MTConnect's `/probe` cache is still dropped, by `InvalidateProbeModel()` on an
  Agent restart. That is now `internal` rather than `private` so the tests which
  used the flush purely to empty the cache drive the real path instead — they
  now cover a case that actually occurs in production. Several unrelated doc
  comments cited `FlushOptionalCachesAsync` as the canonical CAS example and are
  repointed to it.

Tests: the ones that existed only to assert on the removed members are deleted
(3 OpcUaClient, 3 Galaxy, 2 MQTT, 1 MTConnect, 1 AbCip). Worth noting one was
vacuous — `FlushOptionalCachesAsync_resets_footprint_counter` asserted a
footprint of 0 before the flush and 0 after with no discovery in between, so it
would have passed against a flush that did nothing at all.

Full suite green except 7 failures verified identical on master before and
after (4 AbLegacy integration with the compose profile down, 3
Host.IntegrationTests), checked by name rather than assumed.

Docs corrected where they described this as live: `DriverLifecycle.md` (which
carried the #525 placeholder), `drivers/AbCip.md`, `drivers/AbLegacy.md`, and
`v2/driver-stability.md` — whose "polls this every 30 s" + per-driver
`DriverConfig` memory budget were never built as written and are now banner-ed
as design intent rather than description.

Claude-Session: https://claude.ai/code/session_015p7wGqy3YpZNCpDzTpGMKo

tests/Core/ZB.MOM.WW.OtOpcUa.Commons.Tests/Browsing/DiscoveryDriverBrowserFakes.cs

@@ -72,8 +72,6 @@ internal sealed class FakeDiscoveryDriver : IDriver, ITagDiscovery
 
     public Task ReinitializeAsync(string driverConfigJson, CancellationToken cancellationToken) => Task.CompletedTask;
     public DriverHealth GetHealth() => new(DriverState.Healthy, null, null);
-    public long GetMemoryFootprint() => 0;
-    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
 }
 
 /// <summary>An IDriver that is NOT ITagDiscovery — exercises the not-discovery-capable gate.</summary>
@@ -92,8 +90,6 @@ internal sealed class FakeNonDiscoveryDriver(string driverType = "Flat", string
         return Task.CompletedTask;
     }
     public DriverHealth GetHealth() => new(DriverState.Healthy, null, null);
-    public long GetMemoryFootprint() => 0;
-    public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
 }
 
 /// <summary>Configurable IDriverFactory: delegates construction to a supplied func and records what it made.</summary>

tests/Core/ZB.MOM.WW.OtOpcUa.Core.Tests/DriverHostTests.cs

@@ -45,12 +45,6 @@ public sealed class DriverHostTests
         public DriverHealth GetHealth() =>
             new(Initialized ? DriverState.Healthy : DriverState.Unknown, null, null);
 
-        /// <summary>Gets the memory footprint of the driver.</summary>
-        public long GetMemoryFootprint() => 0;
-
-        /// <summary>Flushes optional caches asynchronously.</summary>
-        /// <param name="ct">The cancellation token.</param>
-        public Task FlushOptionalCachesAsync(CancellationToken ct) => Task.CompletedTask;
     }
 
     /// <summary>Verifies that registering a driver initializes it and tracks its health.</summary>
@@ -248,13 +242,6 @@ public sealed class DriverHostTests
         /// <summary>Gets the current health status of the driver.</summary>
         public DriverHealth GetHealth() => new(DriverState.Healthy, null, null);
 
-        /// <summary>Gets the memory footprint of the driver.</summary>
-        public long GetMemoryFootprint() => 0;
-
-        /// <summary>Flushes optional caches asynchronously.</summary>
-        /// <param name="ct">The cancellation token.</param>
-        public Task FlushOptionalCachesAsync(CancellationToken ct) => Task.CompletedTask;
-
         private static readonly TaskCompletionSource CompletedTcs = MakeCompleted();
         private static TaskCompletionSource MakeCompleted()
         {

tests/Core/ZB.MOM.WW.OtOpcUa.Core.Tests/GenericDriverNodeManagerTests.cs

@@ -187,11 +187,6 @@ public sealed class GenericDriverNodeManagerTests
         public Task ShutdownAsync(CancellationToken _) => Task.CompletedTask;
         /// <summary>Gets the current health status of the driver.</summary>
         public DriverHealth GetHealth() => new(DriverState.Healthy, null, null);
-        /// <summary>Gets the memory footprint of the driver.</summary>
-        public long GetMemoryFootprint() => 0;
-        /// <summary>Flushes optional caches in the driver.</summary>
-        /// <param name="_">Cancellation token (unused in test double).</param>
-        public Task FlushOptionalCachesAsync(CancellationToken _) => Task.CompletedTask;
 
         /// <summary>Discovers the address space by throwing an exception.</summary>
         /// <param name="builder">The builder used to construct the address space.</param>
@@ -224,11 +219,6 @@ public sealed class GenericDriverNodeManagerTests
         public Task ShutdownAsync(CancellationToken ct) => Task.CompletedTask;
         /// <summary>Gets the current health status of the driver.</summary>
         public DriverHealth GetHealth() => new(DriverState.Healthy, DateTime.UtcNow, null);
-        /// <summary>Gets the memory footprint of the driver.</summary>
-        public long GetMemoryFootprint() => 0;
-        /// <summary>Flushes optional caches in the driver.</summary>
-        /// <param name="ct">Cancellation token.</param>
-        public Task FlushOptionalCachesAsync(CancellationToken ct) => Task.CompletedTask;
 
         /// <summary>Discovers the address space and registers alarm conditions.</summary>
         /// <param name="builder">The builder used to construct the address space.</param>