chore: add generic DNS record for checking if Coder Connect is running (#17298)

Closes https://github.com/coder/internal/issues/466

```
$ dig -6 @fd60:627a:a42b::53 is.coder--connect--enabled--right--now.coder AAAA

; <<>> DiG 9.10.6 <<>> -6 @fd60:627a:a42b::53 is.coder--connect--enabled--right--now.coder AAAA
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 62390
;; flags: qr aa rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;is.coder--connect--enabled--right--now.coder. IN AAAA

;; ANSWER SECTION:
is.coder--connect--enabled--right--now.coder. 2	IN AAAA	fd60:627a:a42b::53

;; Query time: 3 msec
;; SERVER: fd60:627a:a42b::53#53(fd60:627a:a42b::53)
;; WHEN: Wed Apr 09 16:59:18 AEST 2025
;; MSG SIZE  rcvd: 134
```

Hostname considerations:
- Workspace names, usernames, and agent names can't have double hyphens, so this name can't conflict with a real Coder Connect hostname.
- Components can't start or end with hyphens according to [RFC 952](https://www.rfc-editor.org/rfc/rfc952.html)
- DNS records can't have hyphens in the 3rd and 4th positions, as to not conflict with IDNs https://datatracker.ietf.org/doc/html/rfc5891

tailnet/controllers.go

@@ -16,6 +16,7 @@ import (
 	"golang.org/x/xerrors"
 	"storj.io/drpc"
 	"storj.io/drpc/drpcerr"
+	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
 	"tailscale.com/util/dnsname"
 
@@ -1265,6 +1266,7 @@ func (t *tunnelUpdater) updateDNSNamesLocked() map[dnsname.FQDN][]netip.Addr {
 			}
 		}
 	}
+	names[IsCoderConnectEnabledFQDN] = []netip.Addr{tsaddr.CoderServiceIPv6()}
 	return names
 }