[management] Stop a proxy in the egress path from disabling the check

The dial-time guard reported every non-public address as ErrPrivateHost, which
the credential check reads as "this upstream cannot be reached from here, so
save it unchecked". checkPublicHost has already cleared the target by the time
anything is dialled, so an address refused at the socket is never the
operator's upstream — it is a rebinding attempt, or an HTTP proxy the
management server egresses through. A deployment behind such a proxy would
install this feature and have it silently do nothing on every provider.

It now reports an ordinary failure, which classifies as unreachable and blocks.
Only the resolve-stage check still means "cannot be checked", and that one
knows it is looking at the operator's own host.

This is also why the three fixtures below passed locally and failed in CI: a
sandbox that egresses through a loopback proxy skipped the check entirely,
while CI reached the real api.openai.com and had the dummy key refused. They
want a provider row rather than a working vendor, so they move to a private
address and no longer depend on where a hostname resolves or whether the runner
has egress.

management/server/agentnetwork_realstack_test.go

@@ -101,10 +101,14 @@ func TestAgentNetwork_ProviderCRUD_FansOutToProxyAndClientPeers(t *testing.T) {
 	drain(proxyCh)
 
 	provider, err := agentMgr.CreateProvider(ctx, adminUserID, &agenttypes.Provider{
-		AccountID:   accountID,
-		ProviderID:  "openai_api",
-		Name:        "openai-test",
-		UpstreamURL: "https://api.openai.com",
+		AccountID:  accountID,
+		ProviderID: "openai_api",
+		Name:       "openai-test",
+		// A private address: the save-time credential check leaves it
+		// unchecked rather than spending a dummy key against the real
+		// api.openai.com, which the vendor refuses and which would make
+		// this test depend on the runner having egress.
+		UpstreamURL: "https://10.255.255.1",
 		APIKey:      "sk-test-key",
 		Enabled:     true,
 		Models:      []agenttypes.ProviderModel{{ID: "gpt-5.4"}},