fix: strengthen self-connection guard to check pub_key

[sanity]

Problem

The test_three_node_network_connectivity test was failing in the release/v0.1.39 CI. This failure was caused by the interaction between PR #2140 (self-connection guard) and PR #2142 (response_target fix).

Root cause: The self-connection guard in should_accept() only checked peer_key, which can be None early during node initialization. When the gateway received a connection request that referenced itself, the guard failed to trigger because get_peer_key() returned None.

Investigation findings:

• PR fix: add self-connection guard in should_accept() #2140 alone: test_three_node_network_connectivity passed ✅
• PR fix: use observed external address in ConnectResponse target #2142 alone: passed ✅
• Both PRs merged: failed ❌
• CI logs showed gateway accepting a connection from its own peer ID (self-connection)

Solution

Add a secondary self-connection check that compares pub_key directly, which is always available in ConnectionManager:

// Primary check: compare full PeerId (address-based equality)
if let Some(own_id) = self.get_peer_key() {
    if &own_id == peer_id {
        return false;
    }
}

// Secondary check: compare pub_key directly
// This catches self-connections even when peer_key is not yet set
if *self.pub_key == peer_id.pub_key {
    return false;
}

Testing

• ✅ Added unit test rejects_self_connection_by_pubkey_when_peer_key_not_set
• ✅ All existing unit tests pass
• ✅ test_three_node_network_connectivity now passes locally

Fixes

This should unblock the release/v0.1.39 CI.

[AI-assisted - Claude]