Fix HTTP/3 crash in HQTransaction::_signal_event (#13093)

This fixes a crash where the assertion at HttpSM.cc:2765 fails:

  ink_assert(default_handler != (HttpSMHandler) nullptr)

The crash is triggered when VC_EVENT_EOS, VC_EVENT_ERROR, or a
timeout propagates from the QUIC stream up to
HQTransaction::state_stream_open, which calls _signal_event. The
previous implementation forwarded the triggering Event* as the data
argument to HttpSM::handleEvent. HttpSM::main_handler, however, casts
that data to VIO* and uses it to look up the vc_table entry. Because
an Event* never matches any registered VIO, find_entry returns null
and main_handler falls through to default_handler. When the SM is in
early setup or has already been torn down (kill_this clears
default_handler), default_handler is null and the assertion fires.

This patch passes the appropriate VIO pointer to each handleEvent
call, matching the convention used by _signal_read_event,
_signal_write_event, and the HTTP/2 Http2Stream equivalents. The
original dual-VIO dispatch is preserved so tunnel consumers bound to
the write side still receive connection-level events.

Fixes: #12112

Author: bneradt

src/proxy/http3/Http3Transaction.cc

@@ -353,15 +353,17 @@ HQTransaction::_close_write_complete_event(Event *e)
 }
 
 void
-HQTransaction::_signal_event(int event, Event *edata)
+HQTransaction::_signal_event(int event, Event * /* edata ATS_UNUSED */)
 {
+  // HttpSM::main_handler expects a VIO* as the event data for VC events so it
+  // can locate the vc_table entry.
   if (this->_write_vio.cont) {
     SCOPED_MUTEX_LOCK(lock, this->_write_vio.mutex, this_ethread());
-    this->_write_vio.cont->handleEvent(event, edata);
+    this->_write_vio.cont->handleEvent(event, &this->_write_vio);
   }
   if (this->_read_vio.cont && this->_read_vio.cont != this->_write_vio.cont) {
     SCOPED_MUTEX_LOCK(lock, this->_read_vio.mutex, this_ethread());
-    this->_read_vio.cont->handleEvent(event, edata);
+    this->_read_vio.cont->handleEvent(event, &this->_read_vio);
   }
 }