Subscription EndReason can be lost or mis-attributed

[mtmk]

Surfaced during the OTel metrics PR review (separate from that PR).

NatsSubBase has two independent gates (_endSubscription and
_unsubscribed) and a _exception field that is written before
the reason gate. As a result, NatsSubBase.EndReason can be:

1. Inconsistent with Exception. If a timer or cancellation token
   fires before an exception path runs, _exception is set but the
   reason field still shows Cancelled / IdleTimeout / etc., not
   Exception. The two fields disagree.

2. Stuck at None for connection-driven drain. DrainAsync sets
   _unsubscribed = true directly and never calls EndSubscription.
   There is no Drained / ConnectionClosed enum value.

3. Stuck at None for user-driven DisposeAsync. Same shape: no
   reason is recorded.

4. Silently overwritten by the "first wins" gate when two end conditions
   race (two timers, timer + exception, etc.). The second caller's
   reason is dropped without surfacing the contention.

Possible directions (not prescriptive):

• Prioritise Exception over previously-set reasons when _exception
  is written.
• Add Drained / ConnectionClosed enum values and call
  EndSubscription from DrainAsync and the connection-dispose path.
• Document the "None means user-disposed" convention if that is the
  intended contract.