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

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriver.cs

@@ -64,26 +64,5 @@ public interface IDriver
     /// has a running / not-running distinction worth reporting.</para></summary>
     /// <returns>The driver's current health snapshot.</returns>
     DriverHealth GetHealth();
-
-    /// <summary>
-    ///     Approximate driver-attributable footprint in bytes (caches, queues, symbol tables).
-    ///     <para>⚠️ <b>Nothing calls this (Gitea #525).</b> Its only consumer was <c>MemoryTracking</c>,
-    ///     deleted with the rest of the driver-tier recycle machinery in Gitea #522 — the doc-comment here
-    ///     used to claim Core polled it every 30 s, which was never true in v3. All 12 drivers still
-    ///     implement it; most return a real number, and returning <c>0</c> is equally correct today. Do not
-    ///     read a value from it and act on it without first building a consumer that is actually wired.</para>
-    /// </summary>
-    /// <returns>The approximate memory footprint in bytes.</returns>
-    long GetMemoryFootprint();
-
-    /// <summary>
-    ///     Drop optional caches (symbol cache, browse cache, etc.) to bring footprint back below budget.
-    ///     Required-for-correctness state must NOT be flushed.
-    ///     <para>⚠️ <b>Nothing calls this (Gitea #525)</b> — same reason as
-    ///     <see cref="GetMemoryFootprint"/>: the memory-pressure path that would have invoked it is
-    ///     gone.</para>
-    /// </summary>
-    /// <param name="cancellationToken">Cancellation token for the operation.</param>
-    /// <returns>A task that represents the asynchronous operation.</returns>
-    Task FlushOptionalCachesAsync(CancellationToken cancellationToken);
 }
+