[management] Refuse to pin an agent network gateway onto another account's host (#7519)

An agent network bootstrap stores its cluster as proxy_address, which selects the proxy that serves the endpoint. An account-scoped proxy only receives its own account's mappings, so a pin onto a host another account's proxy declares can never be served, and the endpoint is immutable — a dead gateway until the account deletes its settings. Nothing refused that pin; the domain unique index only arbitrates between endpoints.

Both bootstrap paths now refuse, before the insert, a host that another account's proxy declares, a host another account has labeled pins beneath (self-addressed path), or a hostname that is another account's endpoint (labeled path).

Shared clusters are unaffected: shared proxies are never foreign, and labeled pins under one cluster are never asked about, so any number of accounts still pin beneath eu.proxy.netbird.io. Registration is deliberately unchanged — refusing a proxy for another account's pin would let a pin lock a tenant out after the reaper drops its rows.

management/internals/modules/agentnetwork/manager.go

@@ -1036,6 +1036,15 @@ func (m *managerImpl) bootstrapSelfAddressed(ctx context.Context, settings *type
 	if err != nil {
 		return status.Errorf(status.InvalidArgument, "invalid endpoint: %s", err)
 	}
+	if err := m.requireHostNotForeign(ctx, settings.AccountID, hostname); err != nil {
+		return err
+	}
+	// Another account's labeled pin beneath this hostname makes it their
+	// cluster: a proxy serving them there would never serve this endpoint.
+	// The domain unique index already arbitrates two endpoints on one name.
+	if err := m.requireNotClaimedByOtherAccount(ctx, settings.AccountID, hostname, m.store.HasGatewayClusterPinnedByOtherAccount); err != nil {
+		return err
+	}
 
 	settings.Domain = hostname
 	settings.ProxyAddress = hostname
@@ -1065,6 +1074,16 @@ func (m *managerImpl) bootstrapLabeled(ctx context.Context, settings *types.Sett
 	if err != nil {
 		return status.Errorf(status.InvalidArgument, "invalid proxy_address: %s", err)
 	}
+	if err := m.requireHostNotForeign(ctx, settings.AccountID, parent); err != nil {
+		return err
+	}
+	// Another account's endpoint at this exact hostname means the proxy that
+	// declares it is theirs, so nothing would serve a label beneath it. Other
+	// accounts' labeled pins under the same cluster are not asked about: a
+	// shared cluster carries many of them by design.
+	if err := m.requireNotClaimedByOtherAccount(ctx, settings.AccountID, parent, m.store.HasGatewayEndpointByOtherAccount); err != nil {
+		return err
+	}
 
 	for attempt := 1; attempt <= maxDomainAllocationAttempts; attempt++ {
 		label := labelgen.PickTuple()
@@ -1111,6 +1130,41 @@ func (m *managerImpl) bootstrapLabeled(ctx context.Context, settings *types.Sett
 	return fmt.Errorf("allocate agent network endpoint for account %s: %d attempts exhausted", settings.AccountID, maxDomainAllocationAttempts)
 }
 
+// requireHostNotForeign refuses to pin the account's gateway onto a host that
+// another account's proxy declares. The pin's proxy_address is what selects
+// the proxy that serves the endpoint, and an account-scoped proxy only ever
+// receives its own account's mappings, so such a pin could never be served —
+// and the endpoint it assigns is immutable. Shared proxies are not foreign, and
+// a host no proxy has declared stays pinnable: claiming the address before the
+// proxy's first connection is the documented order.
+func (m *managerImpl) requireHostNotForeign(ctx context.Context, accountID, host string) error {
+	foreign, err := m.store.HasForeignAccountProxyAtHost(ctx, host, accountID)
+	if err != nil {
+		return fmt.Errorf("check proxy host ownership: %w", err)
+	}
+	if foreign {
+		return errHostNotAvailable(host)
+	}
+	return nil
+}
+
+// requireNotClaimedByOtherAccount refuses the pin when another account's
+// gateway settings already claim the host in the shape claimed answers for.
+func (m *managerImpl) requireNotClaimedByOtherAccount(ctx context.Context, accountID, host string, claimed func(context.Context, string, string) (bool, error)) error {
+	taken, err := claimed(ctx, host, accountID)
+	if err != nil {
+		return fmt.Errorf("check agent network gateway claims at host: %w", err)
+	}
+	if taken {
+		return errHostNotAvailable(host)
+	}
+	return nil
+}
+
+func errHostNotAvailable(host string) error {
+	return status.Errorf(status.InvalidArgument, "proxy cluster %s is not available to this account", host)
+}
+
 // isUniqueConstraintError reports whether err is a database unique-constraint
 // violation, matched on the driver message because CreateAgentNetworkSettings
 // deliberately returns the driver error unwrapped.