fix(yamux): handle WINDOW_UPDATE|SYN without reading payload

[arpittkhandelwal]

Fixes #1312

Summary

This PR fixes a bug in the Yamux stream muxer where WINDOW_UPDATE|SYN frames were incorrectly treated as carrying payload bytes.

According to the Yamux specification, the SYN flag can be used with both DATA and WINDOW_UPDATE frames to open a new stream. However, for WINDOW_UPDATE frames, the length field represents the window increment, not a payload size.

Previous behavior:
The muxer unconditionally attempted to read length bytes from the connection whenever the SYN flag was present. When receiving a WINDOW_UPDATE|SYN frame (e.g., from a Lua-based libp2p dialer), the muxer would hang while waiting for a non-existent payload.

New behavior:

• Split the SYN handling logic based on the frame type.
• TYPE_DATA | SYN: Continues to read the optional inline payload if length > 0.
• TYPE_WINDOW_UPDATE | SYN: Applies the length as a window increment and skips the payload reading step.
• Added support for processing FIN and RST flags on SYN frames (opening and immediately half-closing or resetting a stream).

Fixes #1312

Tests

I have added regression tests to tests/core/stream_muxer/test_yamux.py:

• test_yamux_syn_with_window_update: Verifies that WINDOW_UPDATE|SYN frames are handled correctly without hanging and that the window increment is applied.
• test_yamux_syn_with_fin: Verifies that DATA|SYN|FIN correctly opens and half-closes a stream.
• test_yamux_syn_with_rst: Verifies that DATA|SYN|RST correctly opens and resets a stream.

All 25 Yamux tests are passing.