fix(gdb): drain stale debug events before resuming vcpus

When more than one vcpu hits a breakpoint while the VM runs, each sends
a debug event and parks itself in the paused emulation state. The gdb
event loop reports the first and force-pauses the rest, but their
already-queued debug events are never consumed. On the next resume those
stale events remain, so a following `wait_for_stop_reason` dequeues one
and processes it against a vcpu that has since resumed: it marks a
running vcpu as paused, desyncing the pause/resume handshake until the
vcpu threads exit and the event channel disconnects — surfacing as a
fatal `GdbQueueError` ("Remote connection closed" on the client) under a
sustained multi-vcpu breakpoint storm.

Drain the debug-event queue at the start of `resume_all_vcpus`. Every
vcpu is paused there, so none can emit an event and anything queued is
provably stale; dropping it is safe and keeps `vcpu_state` in sync with
the vcpus.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Nikita Kalyazin <nikita.kalyazin@e2b.dev>

Author: kalyazin

src/vmm/src/gdb/target.rs

@@ -219,6 +219,15 @@ impl FirecrackerTarget {
     /// Resumes execution of all paused Vcpus, update them with current kvm debug info
     /// and resumes
     fn resume_all_vcpus(&mut self) -> Result<(), GdbTargetError> {
+        // Every vcpu is paused at this point (all-stop), so it is blocked in its
+        // emulation loop and cannot emit a debug event. Any event still queued is
+        // therefore stale: a sibling that also hit the breakpoint before we stopped
+        // the VM. Drain these now — if left queued, the next `wait_for_stop_reason`
+        // would process a stale event against a vcpu that has since resumed, marking
+        // a running vcpu as paused and desyncing the pause/resume handshake until the
+        // vcpu threads exit (surfacing as a fatal GdbQueueError).
+        while self.gdb_event.try_recv().is_ok() {}
+
         for idx in 0..self.vcpu_state.len() {
             self.update_vcpu_kvm_debug(idx, &self.hw_breakpoints)?;
         }