QUIC: handle ACK_ECN frames (fixes #945)

The QUIC frame dispatcher was missing the case for ACK_ECN frames
(type 0x03), causing connections to be closed with "missing frame
handler" error when clients send ECN counts.

RFC 9000 §19.3 requires implementations to handle both ACK (0x02)
and ACK_ECN (0x03) frame types. Since ngx_quic_handle_ack_frame()
already correctly processes both types, adding the case statement
is sufficient to fix the issue.

Identified and fixed through AI-orchestrated development:
bug identified by need2talk.it founder, root cause analysis and
fix implementation by Claude Code (Anthropic).

Tested in production on https://need2talk.it with HTTP/3 enabled.
The fix follows the same approach as Cloudflare quiche #1053.

src/event/quic/ngx_event_quic.c

@@ -1231,6 +1231,7 @@ ngx_quic_handle_frames(ngx_connection_t *c, ngx_quic_header_t *pkt)
         switch (frame.type) {
 
         case NGX_QUIC_FT_ACK:
+        case NGX_QUIC_FT_ACK_ECN:
             if (ngx_quic_handle_ack_frame(c, pkt, &frame) != NGX_OK) {
                 return NGX_ERROR;
             }