fix(rtmp): chunk decoder multi-chunk reassembly and MSRV compat

The chunk decoder's decode_message() returned Ok(None) after decoding
each individual chunk of a multi-chunk message, causing the caller's
while-let loop to exit before continuation chunks were processed.
When the server sent messages exceeding 128 bytes (the default chunk
size) — such as YouTube's _result response to the connect command
(~191 bytes, split into two chunks) — the second chunk was stranded
in the decoder buffer, the _result was never processed, and the
connection timed out.

Fix: wrap decode_message() in an internal loop that keeps consuming
continuation chunks from the buffer until the message is complete
or there truly isn't enough data for the next chunk.

Also:
- Replace is_multiple_of() with % N == 0 for MSRV < 1.85 compat
- Add full_youtube_server_simulation integration test covering
  the complete Handshaking → Publishing state machine flow

Signed-off-by: Devin AI <devin@cognition.ai>
Co-Authored-By: Claudio Costa <cstcld91@gmail.com>

Author: streamkit-devin

crates/nodes/src/transport/rtmp.rs

@@ -931,7 +931,9 @@ fn process_video_packet(
     counter.add(1, labels);
     stats.sent();
 
-    if *packet_count <= 5 || (*packet_count).is_multiple_of(100) {
+    // `% N == 0` instead of `.is_multiple_of(N)` for MSRV < 1.85 compat.
+    #[allow(clippy::manual_is_multiple_of)]
+    if *packet_count <= 5 || *packet_count % 100 == 0 {
         tracing::debug!(%node_name, packet = *packet_count, %timestamp_ms, %keyframe, "Sent video");
     }
 
@@ -1013,7 +1015,9 @@ fn process_audio_packet(
     counter.add(1, labels);
     stats.sent();
 
-    if *packet_count <= 5 || (*packet_count).is_multiple_of(200) {
+    // `% N == 0` instead of `.is_multiple_of(N)` for MSRV < 1.85 compat.
+    #[allow(clippy::manual_is_multiple_of)]
+    if *packet_count <= 5 || *packet_count % 200 == 0 {
         tracing::debug!(%node_name, packet = *packet_count, %timestamp_ms, "Sent audio");
     }