Split enumeration-miss cause + extract EnumerationFailureTracker + de-duplicate error telemetry

[tyrielv]

What

Extends the directory-enumeration cause tagging from #2071 with a finer split, de-duplicates the error
telemetry, and moves the whole classification/bookkeeping into a new EnumerationFailureTracker class.
No behavior change - metadata and log volume only; the returned HResult is unchanged.

Why

"Directory enumeration failure" ("Failed to find active enumeration ID") is the largest named
regression signature in the field telemetry. #2071 tags the miss as Evicted vs Unknown. Analysis
of that Unknown bucket shows two distinct causes that need to be told apart, plus a per-machine event
storm that inflates the signal:

• The kernel (ProjFS) can deliver a GetDirectoryEnumeration that races the EndDirectoryEnumeration
  for the same handle - a query in flight while the directory handle is closing, or the querying
  process dying mid-enumeration. This is a benign close/query race, not an ID GVFS never held.
• A caller re-enumerating a handle whose Start GVFS lost (for example across a provider restart) can
  re-issue the same failing Get in a tight loop, producing a very large number of error events on one
  machine and drowning the machine-based signal.

Changes

• New EnumerationFailureTracker class. Extracts the classification, once-per-ID de-duplication,
  and the bounded tracking maps into a cohesive, independently unit-testable class (mirroring the
  MissingTreeTracker pattern). It owns all three "why is this ID absent" maps - recently evicted,
  recently ended, and recently reported - and exposes RecordEvicted, RecordEnded, ClassifyMiss,
  and TryReserveReport. It keeps lock-free ConcurrentDictionary state; a coarse lock would
  serialize the hot enumeration path. The virtualizer keeps only the eviction policy (deciding what
  is stale) and delegates the bookkeeping.

• Split the miss reason. EnumerationFailureReason renames Unknown -> NeverSeen and adds
  EndedRecently:

  • Evicted - GVFS's own stale-enumeration eviction removed a live enumeration (self-inflicted).
  • EndedRecently - a Get raced or followed the End for the same enumeration (benign kernel
    close/query race; outside gvfs.exe's control).
  • NeverSeen - GVFS never held the ID: never started, or from before a provider restart (outside
    gvfs.exe's control).
    The virtualizer records an end (and an eviction) via the tracker before removing the ID from
    activeEnumerations, so a racing Get always finds it in one collection or the other. ClassifyMiss
    attributes Evicted (most actionable) first, then EndedRecently, then NeverSeen.

• De-duplicate the error telemetry. The tracker emits the full error once per enumeration ID within
  a window; repeats from a caller's retry loop are suppressed. The first occurrence still logs at Error,
  so the machine-based signal is preserved.

• Bound the tracking maps. The tracker prunes on a throttle from every record point, so no map can
  grow unbounded when one callback (e.g. End) stops arriving - the never-ended scenario this
  instrumentation targets.

Testing

• EnumerationFailureTrackerTests + WindowsFileSystemVirtualizerTests: 44/44 passed.
• Tracker unit tests cover classification, the Evicted-over-EndedRecently precedence, the eviction
  undo (lost-race), the dedup, and the prune/retention of each map. The virtualizer tests cover the
  end-to-end wiring, the record-before-remove ordering, and the precedence through the real callbacks.