[management] Keep an established claim when its re-read is inconclusive

SaveProxy upserts on the proxy ID, so on a reconnect the row Connect just
wrote is the claim the account has held since its first connect, and the
session guard on DeleteProxy matches because the upsert wrote the new
session. Withdrawing that row whenever the post-write re-read errored
surrendered an established claim on a transient store error — a window in
which any other account could take the address — where the pre-existing
code left the row untouched.

An inconclusive re-read still refuses the connect, but marks the session
disconnected instead of deleting the row; only a conclusive answer that the
address is claimed withdraws it. The write-then-re-read argument moves to
the doc of the exported ErrClusterAddressUnavailable, where the API needs
it, and both helpers point there instead of carrying it twice.

Store-backed tests drive the re-read through the real queries — a
reconnect keeps its row, the account's own pin is not a competing claim,
another account's is — since the whole path now depends on the store
excluding the account's own claims.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sa3DsBDP3VciAi4PPG17L6

management/server/store/sql_store.go

@@ -6306,7 +6306,7 @@ func (s *SqlStore) DeleteProxy(ctx context.Context, proxyID, sessionID string) e
 		return status.Errorf(status.Internal, "failed to delete proxy")
 	}
 	if result.RowsAffected == 0 {
-		log.WithContext(ctx).Debugf("proxy %s session %s: no row deleted (superseded by newer session)", proxyID, sessionID)
+		log.WithContext(ctx).Debugf("proxy %s session %s: no row deleted (already gone or superseded by a newer session)", proxyID, sessionID)
 	}
 	return nil
 }