Check roles in OPEN and prevent route leaks with Only to Customer (RFC 9234)

A peer's configured role now does three more things:

- Our OPEN carries the BGP Role capability with what we are to the peer.
  A peer whose own says it is something that does not pair with that, or
  that sends two different roles, is refused with OPEN Message Error /
  Role Mismatch. A peer that says nothing is accepted unless require_role
  is set. A role on a peer in our own AS is a configuration error.
- Routes from a provider, a peer or a route server are marked Only to
  Customer on the way in if they are not already, and our routes are
  marked on the way out to a customer, a peer or a route server client.
- A marked route from a customer or route server client, or from a peer
  with someone else's mark, is a leak and is refused. No policy setting
  turns that off.

The packet crate gains the Only to Customer attribute (malformed:
treat-as-withdraw), and the Role Mismatch subcode is corrected from 8 to
11.

The status page shows whether the peer confirmed its role.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/server/src/session.rs

@@ -123,6 +123,8 @@ pub struct SessionHandle {
     pub ibgp: bool,
     /// What the peer is to us, if that is known.
     pub role: Option<PeerRole>,
+    /// Whether the peer said so itself in its OPEN (RFC 9234).
+    pub role_confirmed: bool,
     /// Whether the peer negotiated RFC 6793 four-octet ASNs. Determines how
     /// AS_PATH is encoded on the wire for this session.
     pub four_octet_asn: bool,
@@ -167,6 +169,7 @@ impl SessionHandle {
             established_at: SystemTime::now(),
             ibgp: negotiated.ibgp,
             role: None,
+            role_confirmed: false,
             four_octet_asn: negotiated.four_octet_asn,
             families: negotiated.families.clone(),
             add_path_receive: negotiated.add_path_receive.clone(),