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/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientMediumFindingsRegressionTests.cs

@@ -13,7 +13,6 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
 ///       <item>Driver.OpcUaClient-009 — WriteAsync timeout maps to BadTimeout, not BadCommunicationError</item>
 ///       <item>Driver.OpcUaClient-010 — OPC UA Byte type maps to UInt16, not Int16</item>
 ///       <item>Driver.OpcUaClient-012 — AutoAcceptCertificates emits a startup warning</item>
-///       <item>Driver.OpcUaClient-013 — GetMemoryFootprint and FlushOptionalCachesAsync contract</item>
 ///       <item>Driver.OpcUaClient-015 — key pure-logic paths exposed for future test expansion</item>
 ///     </list>
 /// </summary>
@@ -97,39 +96,11 @@ public sealed class OpcUaClientMediumFindingsRegressionTests
     }
 
     // ---- Driver.OpcUaClient-013 ----
-
-    /// <summary>Verifies that GetMemoryFootprint returns zero before discovery.</summary>
-    [Fact]
-    public void GetMemoryFootprint_returns_zero_before_first_discovery()
-    {
-        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-mem-013");
-        // Pre-init: no nodes discovered yet, so footprint is 0.
-        drv.GetMemoryFootprint().ShouldBe(0L);
-    }
-
-    /// <summary>Verifies that FlushOptionalCachesAsync completes without throwing.</summary>
-    [Fact]
-    public async Task FlushOptionalCachesAsync_completes_without_throwing()
-    {
-        // FlushOptionalCachesAsync is a cheap no-op before any discovery. Must not throw.
-        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-flush-013");
-        await Should.NotThrowAsync(async () =>
-            await drv.FlushOptionalCachesAsync(TestContext.Current.CancellationToken));
-    }
-
-    /// <summary>Verifies that FlushOptionalCachesAsync resets the footprint counter.</summary>
-    [Fact]
-    public async Task FlushOptionalCachesAsync_resets_footprint_counter()
-    {
-        // GetMemoryFootprint uses _discoveredNodeCount * 512. FlushOptionalCachesAsync resets
-        // _discoveredNodeCount to 0, so a subsequent call to GetMemoryFootprint returns 0.
-        // We drive this through a complete Discovery pass against a stub — instead, test the
-        // contract via FlushOptionalCachesAsync clearing a counter we can observe.
-        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-flush2-013");
-        drv.GetMemoryFootprint().ShouldBe(0L, "pre-flush should be 0 (no discovery)");
-        await drv.FlushOptionalCachesAsync(TestContext.Current.CancellationToken);
-        drv.GetMemoryFootprint().ShouldBe(0L, "post-flush should still be 0");
-    }
+    //
+    // Three tests lived here covering GetMemoryFootprint + FlushOptionalCachesAsync. Both members
+    // were deleted from IDriver in Gitea #525 as unconsumed surface, and these went with them. Worth
+    // noting one was vacuous anyway: it 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.
 
     // ---- Driver.OpcUaClient-015: pure-logic paths ----