feat(election-iomgr-refactor): add on_connected fire-once callback to endpoint_t

Add void (*on_connected)(endpoint_t *ep) field to endpoint_t with
endpoint_set_on_connected() helper. io_handler fires it exactly once
immediately after ssl_handshake_done=1 then nulls the pointer.

Enables ha_start_election_async to send OP_requestVote as soon as
the TLS handshake with a peer completes — no blocking SSL_connect.

6 new unit tests pass; controller suite 1/1 OK.

src/lib/testsuite/test.c

@@ -35,6 +35,7 @@ extern void test_hooks(void);
 extern void test_cluster(void);
 extern void test_cluster_parse(void);
 extern void test_libxce_retry(void);
+extern void test_netio_on_connected(void);
 
 void setUp(void) { return; }
 
@@ -139,6 +140,7 @@ int main(int argc, char* argv[])
     TEST(test_cluster);
     TEST(test_cluster_parse);
     TEST(test_libxce_retry);
+    TEST(test_netio_on_connected);
 
     return UNITY_END();
 }