network: re-detect is_proxy_tor every time we connect to a server

This avoids some false negatives for is_proxy_tor.
(previously we only set is_proxy_tor when the proxy settings were changed)
In particular, consider scenario:
- Tor browser not running
- user sets "localhost:9150" as proxy
- detection sets network.is_proxy_tor to False
- user starts Tor browser
- network, due to retries, finds proxy working and connects to some servers
- network.is_proxy_tor remains False

electrum/network.py

@@ -648,25 +648,28 @@ def _set_proxy(self, proxy: Optional[dict]):
 
         self.logger.info(f'setting proxy {proxy}')
         self.proxy = proxy
+
+        # reset is_proxy_tor to unknown, and re-detect it:
         self.is_proxy_tor = None
+        self._detect_if_proxy_is_tor()
+
+        util.trigger_callback('proxy_set', self.proxy)
 
+    def _detect_if_proxy_is_tor(self) -> None:
         def tor_probe_task(p):
             assert p is not None
-            tor_proxy = util.is_tor_socks_port(p['host'], int(p['port']))
+            is_tor = util.is_tor_socks_port(p['host'], int(p['port']))
             if self.proxy == p:  # is this the proxy we probed?
-                self.logger.info(f'Proxy is {"" if tor_proxy else "not "}TOR')
-                self._tor_probe_done(tor_proxy)
+                if self.is_proxy_tor != is_tor:
+                    self.logger.info(f'Proxy is {"" if is_tor else "not "}TOR')
+                    self.is_proxy_tor = is_tor
+                util.trigger_callback('tor_probed', is_tor)
 
+        proxy = self.proxy
         if proxy and proxy['mode'] == 'socks5':
             t = threading.Thread(target=tor_probe_task, args=(proxy,), daemon=True)
             t.start()
 
-        util.trigger_callback('proxy_set', self.proxy)
-
-    def _tor_probe_done(self, is_tor: bool):
-        self.is_proxy_tor = is_tor
-        util.trigger_callback('tor_probed', is_tor)
-
     @log_exceptions
     async def set_parameters(self, net_params: NetworkParameters):
         proxy = net_params.proxy
@@ -904,6 +907,9 @@ async def _run_new_interface(self, server: ServerAddr):
         self._has_ever_managed_to_connect_to_server = True
         self._add_recent_server(server)
         util.trigger_callback('network_updated')
+        # When the proxy settings were set, the proxy (if any) might have been unreachable,
+        # resulting in a false-negative for Tor-detection. Given we just connected to a server, re-test now.
+        self._detect_if_proxy_is_tor()
 
     def check_interface_against_healthy_spread_of_connected_servers(self, iface_to_check: Interface) -> bool:
         # main interface is exempt. this makes switching servers easier